Aphelios has the privillage of swapping between every keystone in the precision tree with grace, and here's why:

Press The Attack
Allows you to have a strong lane presence, and allows you for a cheesy strat with Callibrum (green sniper). You can autoattack twice, then hit a Callibrum Q, and proc a long range, third PTA attack that deals increased damage. Its worth mentioning that almost all of Aphelios's Q's can crit and proc On-Hit effects (Press The Attack included)

Lethal Tempo
Aphelios has great synergy with high attack speed steroids, he also procs and keeps up his Lethal Temp with ease. Imagine triple Infernum autos that crit for 252,5% of your AD, spewing out every half a second. Pretty cool

Fleet Footwork
Gives you good amount of healing and movement speed, considerably weaker than all the other choices but still viable if you think you will have no lane presence at all, and will be poked often

Conqueror
Mine, and many others favourite choice of keystone, gives you big chunk of AD and good sustain in longer fights/teamfights. It may seem hard to stack and keep up at first, but the amount of range and AOE (especially with Hurricane) that Aphelios has nearly completely eiminate these problems. Even in lane, conqueror can be almost immidiately stacked with your Severum (Red Pistol) Q.

Other rune choices have little reason to be swapped, except for the choice between Triumph / Overheal and Inspiration / Sorcery secondary rune tree

Triumph
Has saved many people countless times, extremely valuable in teamfights but almost unnoticable in lane. Pick if you are confident in your ability to survive lane and excelling at teamfights and small skimrishes.

Overheal
May seem insignificant in the early game on all of the other markspeople, but on Aphelios its a whole different story - because you start with Severum (red pistol), that has a built-in % scaling lifesteal (starting at 3%), AND a built in overheal on top of that (Severum is absolutely insane), you can get quite a sizable shield even at level 1 (you can stack it up on the first wave/while leashing your jungler). It can tank up to hundrets of damage over the course of ealry/mid game, and skyrockets in the late game with massive shield amounts. Pick Overheal if you want more presence in lane, and additional scaling into the lategame.

Its also important to note how well Aphelios abuses Legend: Bloodline - most other ADC's would opt for Legend: Alacrity, as bonus attack speed is beneficial to almost any Marksperson out there (looking at you, Jhin). But Aphelios, having bonus attack speed gain build into his passive, can freely spec into the scaling lifesteal of Bloodline, and benefit even more from it in the lategame than he would from Alacrity.

The [Sorcery / Inspiration] Dilemma

Sorcery -->Nimbus Cloak + Gathering Storm
Nimbus Cloak & Gathering Storm are what I like to refer to as "The Supersonic Combo", as it grants you high speeds after casting any summoner, allowing Aphelios to have some form of mobility (which his kit has none of, besides of the movement speed on Severum Q), and granting him supersonic scaling with Gathering Storm. Pick these two if you want to play for late game, and the enemy team has champions which can easily collapse on you.

Inspiration --> Magical Footwear + Bicuit Delivery
These two choices allow you to save money for your big items faster. Magical Footwear grants you free boots with increased movement speed, allowing you to kite better and save up to 350G in value. Biscuit Delivery on the other side, encourages short trades in lane, as you can heal up some of the damage you will suffer while doing so. Pick these runes if you want more lane presence and want to reach your spikes faster.

1. [IE --> ER --> --> RH] ; If you are snowballing hard and want to dish out as much damage as humanly possible as early as you can. IE + RE gives you 150 AD and CDR for spamming your abilities (helps you swap your weapon queue). Build when ahead

2. [ER --> IE --> RH] ; Same as the above, the only difference is that Essence Reaver has cheaper and more accessible compoments and is cheaper overall. Build when even/ahead

3. [ER --> RH --> IE] ; Building in that order makes you spike faster, at the cost of the sheer power of the spike. Build if even/behind

4. [IE --> RH --> ER] ; Same as (3.), but you trade the easy to build compoments of ER for a bigger spike with IE. Build if even/ahead, and in need of AOE damage

The point of Aphelios's lategame itemization is to give him survivability - at this point in the game you already pack a punch, and these items should give you the durability to do so even better.

Bloodthirster - Gives you an additional 80AD to your already massive damage caoabilities, and provides great sustain (when paired with crit an Runaan's Hurricane you can basically heal to full HP with two autos.) I recommend building this as your 4th item 90% of the time.

Guardian Angel - Gives you another chance after you misposition and die a horrbile death, additionally providing a chunk of armor for these pesky AD assassins. It's important to mention that an early stopwatch to prevent getting oneshotted Is also recommended, as it costs only 650G.

Phantom Dancer - You can build it as a 4th item for a cheap and accessible movement speed and attack speed buff, shield for survivability, and that nasty 100% Crit Cap. I would reccomend building it as the 5th item though, because Bloodthirster gives you an even bigger mid/late game spike.

Okay, I’m no pro at our moon baby, but I’m still M7 60K + on him so maybe I will help you with some fancy tips

1. As Aphelios, you have very limited range of mechanics, but that doesn’t make him any easier - less fancy stuff to dodge with makes him harder to play than other carries. The first thing you gotta remember is your spacing and positioning. One misstep -> your dead, and season 11 will make that statement even stronger because of the increase of the overall damage. I recommend playing customs 1v5 bots, it may not be hard to outplay them, but will force you to control the whole map at once (to not get out pushed) and will force you to position better (if not, you get slaughtered by 5 bots)

2. My next point is connected to the first one, and it touches on kiting. It’s an essential skill to have on all adc’s, but aphelios takes it to the next level in couple of different manners. First, you gotta remember that Phel has legit no movement capabilities outside of his Severum (red gun) Q, that gives him movement speed, so kiting should be your top priority to master on this champ. The second thing that’s unique to Aphelios however, is the fact that he has no “E” ability (as in, while playing Aphelios pressing E does not do anything). But here’s the twist - you can bind your attack move to E (I’ve heard of many people that find it extremely comfortable), which will result In you not forgetting about using E on other champs (seems stupid, but muscle memory does it’s own thing), and will let you adapt to a very important skill of attack moving (that is useful on every ranged champion)

3. The next tip is a very simple and easy one - if you wanna swap weapons (W) mid fight, try to do so right after autoattacking. This will sync the W animation with your AA recovery, resulting in a fluent weapon swap that doesn’t interrupt the flow of the fight.

4. Another tip touches on weapon management, and let me tell you this **** ain’t easy - I’ve lost many fights just because of having bad weapons that were not suited to the situation (or I just plain old ran out of ammo mid fight and got locked in the weapon replace animation for a second -> death). The key to mastering your weapon management is quite simple, yet hard to perfect - you simply have to watch over the map and map presence of the enemies constantly, and try to cycle your weapon queue accordingly. They seem to be doing baron and you have no chance in an all in? Try emptying your gravitum (the purple gun that would be useless in this scenario) on the raptor camp, and swap to a more favorable Callibrum + Cresendum (green sniper and silver chakra), that may lend you a stylish steal with a sneaky turret over the pits wall. They doing baron but you have a chance at winning? Swap that callibrum, shooting it’s Q out at thin air, to get that nasty 5 man Infernum Q+R+Aa combo, that may single handedly win you the game with its disgusting aoe.
Here's a useful weapon combo chart.

5. And lastly, I personally think that you shouldn’t be afraid of taking over the lane as early as lvl.2. It may seem like an Int, trying to be dominant in lane with a late game hypercarry, but the fact that you start with Callibrum (green sniper) let’s you outrange 90% of botlaners through levels 1-3 (you’ll likely run out of callibrum ammo soon after). Certain matchups require certain mentalities and weapon combos, but you gotta remember one, most important thing - you play to have fun and enjoy yourself, not to win, and even a screwed lane can be turned into a great pop-off, if you give yourself enough time and farm.

Thanks for reading all this **** lol