basically senna has a 0.3 attack speed ratio, instead of 0.625, which means any attack speed you buy is less than half as effective as it should be. if you buy a dagger, you are throwing away about 150 gold for no reason.
sennas crits deal 170.5% ad or something, so if you build crit, you deal as much damage as other people who build crit. but her non crits deal 120% ad, plus you get crit passively as you get mist. So you can afford to build non crit items, reap the rewards of that(tankiness/lethality), and be less punished for not building crit cuz you do 20% ad damage on hit. But if you build crit, you just deal the same damage as other adc's with no reward. If you build non crit, you get everything those items give, plus 20% ad on hit. Basically, her passive rewards and encourages you to avoid building crit. Additionally, if you overcap on crit, it converts to lifesteal. But the rate it converts it at is really inefficient, so its better if you avoid that by not building crit
Since building attack speed and crit limits her build options so much, it only really leaves lethality and bruiser items. Which is fine! senna makes it work, she a G.
You can also buy infinity Edge without buying any other crit, as long as you have 80% crit it is efficient. You can get IE second or third item, but you can also get it later, it doesnt matter, just get it at some point in the game. IE will make your crits deal like 195% ad or something
Also dont fall for rageblade, they removed the infinite on hit damage from sennas mists crit, lowered the damage equal to how her crits are nerfed, plus the more ad you have, the more bonus damage an IE crit will do compared to a rageblade on hit, which only scales with crit chance. and senna kinda, gets infinite ad from her passive, a lot more ad than crit. When you hit a certian amount of mist or ad, rageblade just makes you deal less damage than if you were to sell it.
So uh yeah i think thats everything, stop building attack speed and crit its cringe asf stop doing it

uh early game u should harass with your very large range to get mist and deal damage to scare them and if u have a strong early game sup, kill them . u dont wanna get too aggresive early though, cuz you have really low base hp to compensate for ur heal. basically its easier to get burst out, so dont let that happen. go in when their cc is on cd n stuff.