Focalors gives healers more jobs. Corrosion and the serpent knights do too.
Food gives another way to heal. HSR Simulated Universe allows food to be used after reaching a certain milestone.
Some healers scale with HP while others scale with ATK. For example, Barbara's Q heals for 31.68% Max HP + 3727 at lv10. By comparison Charlotte's scaling is 461.83% ATK + 3539. Supposedly the two should heal a similar amount.
Ignoring the residuals and only focusing on the hp/atk scaling parts, 461.83 is around 15 times more than 31.68, so here we see atk scaling numbers being 15 times more than hp scaling numbers to give a similar amount of healing. As explained in other articles, atk scaling is usually 10-15 times more than hp scaling to achieve similar dmg. We say 10-15 times to account for attack buffs like Bennett (which can buff atk by around 1.5 times). If an atk scaler uses Bennett and deals x atk scaling damage, then the equivalent hp scaling is x/10. If an atk scaler does not use Bennett, then the equivalent hp scaling is x/15. So for healers, it seems Charlotte follows the 15 times rule. The reason could be if one is using Charlotte in the team, one may not necessarily be using Bennett as well. Or even if one is using Bennett, one may not be casting Charlotte's Q within Bennett's Q. However, if one actually does cast Charlotte's Q in Bennett's Q, then Charlotte can technically heal more than Barbara (1.5 times more, simply because atk scalers can benefit from more atk buffs than hp scalers). If an hp Bennett ever releases, maybe this would change.
Apparently Yaoyao also heals a lot with her Q. You need to jump and run around with her.
Note Jean's Q gives instant healing of 477.28% ATK + 3,658 at lv10, which appears less than Charlotte's Q. However, Jean's continuous regeneration 45.22% ATK + 338 per second (and lasts 10s) is more than Charlotte's 16.59% ATK + 126 (which lasts 4s).
This gives Jean pretty good synergy with Furina (in say Xiao-Jean-Furina, vape, taser, freeze, mono-hydro, etc.).
So how much is actually healed? We assume 30k hp and 2k atk. Barbara heals 13231 to each party member at lv 10. If a typical (non hp scaling) character has around 20k hp, and hp scaling characters have around 30k hp, then Barbara can heal around half of their hp. And she does this for the whole party, so her heal can give 200 points to Focalors' buff (50% dmg bonus). The remaining 100 points can be obtained via Furina's E drain or a destruction character (Xiao, Wrio, etc.).