Qiqi (七七) is a zombie healer that despises Hu Tao. Why does Qiqi accompany Baizhu at Bubu pharmacy? The dark truth is that Qiqi could refer to Qi Tai, who was executed by Zhu Di during the Ming dynasty. That is actually kind of sick, cruel, and disgusting.
Her healing scales with her ATK. She can heal through her E, Q, and passive.
Her E lasts 15s and has 30s cd, giving her quite a lot of downtime. Thus sac sword would be helpful. While her E is active, Qiqi's NA/CA will heal the whole party (good for Furina teams). Moreover, the active character will be healed periodically.
Her Q marks opponents. When a character deals dmg to a marked opponent they get healed (ST healing).
Ocean Clam (released in Version 2.3) 4 piece is her bis. Before that Noblesse Oblige or even Blizzard Strayer could also work.
ATK/ER sands, ATK/Cryo DMG goblet, Healing bonus circlet. Substats: ATK, ER > CRIT. More ATK and healing bonus means more healing and bubble damage. ER is for getting her 80 cost burst more easily and if one already has enough ATK. Because she can crit (unlike Kokomi), crit substats can sometimes be ok for additional damage. However the bubble damage does not crit. CRIT and Cryo DMG are really for her burst damage (which actually has decent scaling). But to maximize her healing specifically (and bubble damage), ATK goblet and Healing bonus circlet are usually preferred. Note she gains healing bonus upon ascension.
Sacrificial Sword is good for resetting her E's cd. Any sword that gives a lot of attack is also good (such as Aquilla Favonia). ER swords such as Skyward or Favonius are also good.
She can be a healer for pretty much any team (except maybe Hu Tao teams).
Since she is usually run with the Ocean clam set, pairing Qiqi with an Electro applier such as Raiden, Fischl, or Beidou will proc Superconduct, reducing Physical RES by 40%, and increasing her bubble's damage. In fact, Raiden is one of Qiqi's best teammates to trigger Superconduct and charge up her 80 cost burst.
Because of this synergy, Qiqi can be used in Eula-Raiden teams for Cryo resonance and healing.
Qiqi can also be used in a taser team: Qiqi-Beidou-Fischl-Xingqiu where Qiqi is the on field auto-attacker and Beidou-Fischl provide off field Electro DMG.
She can also drive hyperbloom/fridge teams (e.g. Qiqi, Nahida, Furina/Yelan/XQ, Raiden). Because her E can give a teamwide heal (when Qiqi is on-field) she can synergize well with Furina.
Qiqi and Baizhu can both heal the party on E. Qiqi just needs to be on field to do it, and she may have 15s downtime without sac sword. By contrast, Barbara, Charlotte, etc. heal the party on Q and thus want ER. Although Baizhu's heal is on E, he may not heal as much as a burst healer (see Barbara for scaling comparison), which can result in fewer stacks for Furina's Q.
Charlotte Q has 461.83% ATK + 3539 healing at lv10. Qiqi E has 19.01% ATK + 148.31 healing per NA/CA at lv10. So if Qiqi uses around 25 NA/CA during her 15s E, she can heal as much as Charlotte's instant button press. But doing 25 NA/CA takes time and may not cap Furina's buff immediately, especially since one is spending on field time with Qiqi (rather than with a destruction character like Wrio/Xiao that can self-drain hp for more stacks). The conclusion is that among healers, having an instant burst healer tends to give the most stacks for Furina quickly.
And let's not forget that Jean can heal the team with NA (no downtime), making her synergize well with Furina too. Jean's NA have 50% chance to regenerate 15% atk as hp for party members. So on average each NA heal 7.5% atk for party. Qiqi's E healing has around 2.5 times more scaling, but may have 15s downtime and requires E, whereas Jean's team healing is on NA without E. Of course Jean's Q can heal the party too. So Jean can be good in say a freeze team such as Wrio-Furina-Jean where she is used as a burst-healer (rather than on-fielder).
For more info see healing.