New IPhone; How can I prolong battery life?

[YourJoyWillTurnToAshes]

I've seen conflicting advice on this subject and I just wanted to see if anyone can give me some tips about charging my IPhone to prolong the battery life as I used to charge it as and whenever I needed it and my old phone used to shutdown at 30% charge.

The tips I've heard are;

1. let the battery run down completely and then charge it to 100%

2. don't let the battery get below 50% charge and top it up after that

I don't use my phone whilst it's charging and I don't charge it in the case but I'm confused; number 1 or number 2?

3. It doesn't matter. Modern phones have modern batteries.

Back in the 1980's people used Nickel Cadmium batteries which had "memory effect." Now, they don't.