Master Yi is one of the best hypercarries in the game he can 1v9 games with a good jungle strategy, even though not strong in the early game, you should look for level 4 ganks because most of the times it's just a free kill. in the mid game he is probably the strongest nungler in the whole game. in the late game he kinda falls of due to cc, dashes and zhonya's.

Now let's look at the items. In ny opinion the strongest build in the game is my display buuld. Kraken Slayer is the best first item for Master Yi since its damage application every third attack and its clear speed due to noonquiver. Master Yi always goes defensive boots, either tabies or mercs. The best second item on Master Yi is Blade Of The Ruined King, ir shorlty "BORK". BORK provides you with extra damage and attackspeed plus its passive that slows people after some attacks. Steraks is in most games necessary due to high burst and cc opponents, its passive gives a well needed shield which is crucial for staying alive in fights. Death's Dance heals you after every kill, it is ABSOLUTELY A MUST BUY ITEM since without it you will probably not live enough in fights resulting in your team losing, Wit's End is really good with mage opponents with its magic resistance, his onhit passive gives you extra damage per attack and synergizes really well with BORK and Master Yi's own passive.

Master Yi's playstyle revolves around his auto attacks, thus good kiting is a necessity. His Q (Alpha Strike) is a quite tricky rather than simple ability. Alpha Strike is the best ability to dodge skill shots and tower shots. Timing your Q is a hard thing and takes time to master. In dives you should always tank the first two turret shots and then Q the opponent to dodge the third and kill him shortly after.
Master Yi's W is a healinf and damage reduction ability. Makes it great for doing the so called meditate bug which blocks most of the turret's shot (look up in yt: cowsep meditate bug, he explains it the best).Its W is also a great auto attack resetter, which means you can autto>w>autto and deal double the amount of dmg in a really short time period. Master Yi's E is just true damage applied nothing to really talk about. Last but not least his ultimate. His R allows Yi to run faster and not get slowed in the process, Every kill you get with his r still on the duration extends. Be careful though, initially his r doesnt last long so be careful when you use it