35w or 20w or 12w

First time Iphone user here. After 10 years of using android, I finally decided to try iOs. Bought one and got freebies.

Questions: 1. is it safe if I use my old android charger (12w) 2. should i buy another (20w) and use that which some suggests? 3. Should i use the freebie charger (35w)? 4. How does it affect my battery?

Thank you kind peeps😃