Passive - Every 4th attack you do a double strike, when you are going to engage somebody you can stack your passive first and then attack, especially good when you are in a lane.

Q ability - This ability is great at diving people, getting away or chasing somebody. Most of the time you want to use it instantly when chasing somebody, but when diving or fighting against somebody with cc you wanna save it up. When you dive in somebody's tower you can hit the enemy champ and use q to take not any tower damage. You can also dodge moves with your q, this is very handy against people with cc abilitys like morgana and lux.

W ability - This ability is handy to reduce dmg (it also reduces turret dmg), it's at his strongest till level 6, you can stall time for your teammates to come or for your ability's cd to come off.
Besides that while holding your W ability it will charge your passive.

E ability - It's not a special ability you do true damage, just use it while engaging somebody.

R ability - Great for chasing people or running away, because you cannot be slowed and can deal a lot of more dmg(you get increased attack speed). Your ult also decreases your other ability's cds. It's duration will increase with every kill/assist.

Picks:
Squishy's are the best picks for you like lux and missfortune, but you can also be a tank shredder with the right build.
You are a late game 1v1 demon, so atp if you play well you can win almost every 1v1.

Counter's for yi:
Champ that have a lot of cc are very strong against yi for example pantheon, reksai, nunu & willup and ect.
But champs that can get untargetable are also strong against yi, for example elise.
But in the end it also depends on the player, you can dodge a lot of cc with your q thus winning the 1v1.

Crit build - this build can be very strong, but only if you are snowballing/not make any unnecessary deaths, when you get behind it's almost impossible to get back. So I recommond only using this build if you think you are good enough with yi/the match up is in favor for yi.
Crit build/my own build is my favourite build tho, it's so satisfaying to see the enemies hp getting deleted.

Standard build - This build is just strong, kraken slayer and rageblade are you biggest power spike, you will do consistent dmg and when you are behind it is still not favorable, but you can get a comeback more easily.

My own build - This build is sooooo insanly strong when snowballing, but it isnt as good as the standard build, its the same with the crit build, when falling behind you just can't get back up. It's also in end game not as strong as the other builds.
But if you wanna have fun, try this build.

Info - I have to say, a lot of builds works on yi, he is very versatile. So in the end it doesn't really matter. You can even mix up build and try them