I would love if this was "fine-tunable" on iOS. On macOS there are various ways to cap the maximum charge percentage. On iOS (and macOS by default) it tries to learn your charging habits and do various things based on that, but it never actually caps you to 80%. For example, it knows I plug my phone in over night, so it charges to 80%, pauses, then does the remaining 20% right before I wake up.