The advice I have gathered over the years reading iPhone/iPad batteries and my personal experience can be summed up as follows:
1) For a good battery health, it needs to be exposed to different charge/discharge levels, so fluctuate regularly between 90% and 15-20%. Do not be afraid to let your battery run low. Aiming to keep it at a 100% is not a good strategy. However, if you charge it slowly (see below) at night and it gets to a 100%, it is totally fine as the device will just stop taking power in once charged completely.
2) It is better to charge a Lithium-ion battery with the least powerful charger possible. This is the reason Apple still ship iPhones with those “weak” chargers in the box. Sure they take longer to charge your device to a 100%, but are much better for the battery health.
3) When you have a choice, charge it in a cool/cold environment. Attaching the phone to a windscreen of a car (heat exposure) and having it plugged into a charger at the same time is really bad for the battery!
Having said that, Apple are continuing improving the power/battery management on their latest iPhones and iOS 13, which should help.