Yoimiya

5★

PyroBowMain Damage

TL;DR

Yoimiya's Skill sets her arrows on fire for 10 seconds. You stand there and tap normal attacks until it runs out, then swap off. She's a bow character who never aims: the whole build, the whole team and the whole rotation exist to make those 10 seconds bigger.

Rotation

Burst (Q) first, if you have the Energy for it.
Swap out and let the rest of the team set up, Hydro last.
Come back and press Skill (E). A ring of sparks appears around her feet.
Tap normal attacks for the full 10 seconds.
Swap off when the sparks go out.

Abilities

Skill (E)	For 10 seconds her normal attack arrows turn into Pyro and hit a lot harder. Cooldown is 18 seconds. It switches off the moment she leaves the field.
Normal attacks	A five shot string, fired by tapping rather than holding. This is where nearly all of her damage comes from, in and out of the skill window.
Burst (Q)	60 Energy, back in 15 seconds. Marks one enemy for 10 seconds. While that mark is up, every hit your other characters land on that enemy sets off a Pyro explosion, once every 2 seconds. It also hands the rest of the party an Attack bonus of 10% or more for 15 seconds.
Charged attack	Her aimed shot. Fine for shooting things off walls, no part of her damage. The skill turns off the homing arrows a full charge would normally give you.

The one mechanic to understand

The 10 seconds are hers, start to finish.

The skill state ends when she leaves the field, not when the timer does. So there is no swapping out for a quick Hydro application halfway through and no popping back for someone's burst. Everything the team does happens before she presses Skill (E). On top of that, every arrow that lands adds a small Pyro damage bonus that stacks up to ten times, so the back half of the window is worth more than the front. Cutting it short costs you the good part.

What actually matters

The two or three real decisions. Everything else is noise.

Level her normal attacks first. Skill and Burst are a long way behind. Both talents just change what the arrows do.
She wants a Hydro character. Fire into a wet enemy is roughly double the damage. Her arrows land often enough to keep it coming. Without one, her arrows are just fire.
The Burst is mostly for the team. The Attack bonus skips her. The explosions only fire off your other characters' hits, so it's something you press on the way in rather than damage you're waiting on.
Energy Recharge is not a stat she needs. 60 Energy for a buff she doesn't get is a bad trade against more Crit. Press the Burst when it happens to be up.

Gear

A build that works. KeqingMains lists every other option.

Weapon	Crit and Attack, plus anything that boosts normal attack damage. Rust is a standard banner 4-star built for exactly this. Hamayumi is craftable if Rust hasn't turned up.
Artifacts	4-piece Shimenawa's Reminiscence. It adds 50% normal attack damage for 10 seconds when you press the skill, which is the window you care about. It eats 15 Energy each time, so the Burst gets rarer. That's a fair price here.
Main stats	Attack sands, Pyro damage goblet, Crit circlet. Swap the sands to Elemental Mastery if she's paired with Hydro.

Works well with

Hydro

Anemo

Pyro

Hydro is the one that changes her numbers. Anemo pulls the enemies into one place so a stationary shooter can hit them all. A second Pyro is there for the Attack buff and the resistance shred, not for reactions.

Don't bother

Aimed shots inside the skill window. The bonus is on normal attacks only.
Energy Recharge past whatever your artifacts happen to roll.
Swapping out mid-window to squeeze in a teammate. The fire goes out with her.
Physical damage builds. Her arrows are Pyro for the part of the fight that counts.

Want the full theorycrafting version? Read Yoimiya on KeqingMains.