MacBook Air - new battery process?

Hi, I got my MacBook in 2020 and the battery is showing signs of dying. I just made an appointment for next Saturday. I'm curious what to expect. I guess the store will ship it offsite where they can replace the battery. There's no need to access my files, right? Can they log in with an Admin password to access my files to test the new battery? I do have my files backed up on the G-star portable thing. If that's what you call it LOL

Does anyone know the price of a battery for a 2020 model? Or I guess one battery fits all models, new and old?

Is there an extra fee for shipping it to the repair center, or do they repair it at the store?

I know I can ask these questions at the appointment but I am anxious and curious. Thanks in advance.