Overview
Venti is popular because of her high-risk, high-reward playstyle. The lower HP she is, the more damage you do, so a certain level of skill is required to play this character effectively. But those who do are rewarded with one of the best characters in the game.
Elemental Burst: Wind's Grand Ode
Venti fires off a Stormeye at a fixed distance away from himself. It will suck in enemies in a large radius. If the Stormeye comes into contact with Pyro/Hydro/Electro/Cryo, it will absorb that Element and deal additional Elemental DMG of that type.
The best crowd controller in the game. Be careful though, some characters can't hit enemies when they are lifted up by his ult.
Constellations
C1: Splitting Gales
Terrible constellation, completely useless unless you're going for a meme build.
C2: Breeze of Reminiscence
Decent damage increase, but nothing too impressive.
C3: Ode to Thousand Winds
+3 to his Burst. Pretty good for some extra damage.
C4: Hurricane of Freedom
Pretty useless unless you use Sacrifical Bow on him.
C5: Concerto dal Cielo
+3 to his Skill. Good for extra damage.
C6: Storm of Defiance
Not a bad constellation, but a terrible C6. There isn't a point to C6-ing him, unless you're a crazy whale.
Teams
Venti is a great Support and can fit on tons of teams. He's the best with Swirl Teams, but he can fit almost anywhere.
Venti + Ganyu + Mona
Venti + Childe + Fischl
Venti + Kokomi + Kazuha
Venti + Kazuha + Bennett