hi guys, wanna ask about changing iphone battery.
according to switch website or machines, they could take a day or more if required.
another point is they will do a factory reset.

so my question is:
1. why do they need to do a factory reset?
2. after reset, what will happen to all my phone data?
3. itunes is no longer supported by new windows. how do i backup my iphone fully or most of it? especially contacts and settings?
4. how reliable is 3rd party battery?

thanks.