Optimum iPhone 13 Charging Procedure

I would like to know which is the best charging procedure to maximise battery life on the iPhone 13.

l have searched for advice on the internet found many conflicting views and I am getting more and more confused, for example:

1) Is it ok to charge to to 100% or should I only change to 70%, 80% or 90%, which is optimum ?

2) Is it ok to let the charge drop below 20% ?

3) Is it detrimental if I charge when the iPhone is between 40% to 70% charged ?

What is the optimum charging regime ?

Thanks in advance.