Berlin is easily navigated with card payments. My sons (we live in Munich) manage nearly exclusively with apple pay linked to their own debit card. The only places they need cash is for the Döner (which costs 6.20€) at a place they love.

Funny anecdote: my colleagues son (16) went to London last year and we organized a debit card for him at the last moment via Revolut so he could pay for food during the school trip as everything in London has become cash free. In the end the class had to eat at McDonalds each and every time because some classmates only had cash with them and McD was one of the few outlets that still accepts cash