The basic combat system in Genshin heavily depends on elemental (chain) reactions. Memorizing the elemental system is the key to successful late game battles.
Every character has a standard attack, a charged attack and a plunge attack (aerial launch), followed by one active skill and an elemental burst (ultimate).
Mind that you cannot use as much healing items as you like during combat. Every character has a limit of how much they can "eat" until you cannot heal them with items anymore. So relying on one healer isn't that bad. They added this mechanic in v0.9.9.