How would I use background app refresh to update the content of my app?

How would I authenticate Game Center and local player in my app? I am trying to add achievements, but the whole “authentication” part is very confusing to me.

I am wondering how I would let my app share data, in this case a number, that is on one user’s screen to a different user. I want parents with accounts on my app see the amount of points their child with a different account has. Please help!

i want my app to do this: when a button is pushed, it will add the specified number and the number that is in the label then show the total of the numbers in the label. Please help!

When I create an archive on Xcode, and then distribute it, it loads, but then when it starts to upload to App Store connect, it ends and there is an error that says “could not find main class” then lists a bunch of Java files. Please help!

How would I add Game Center achievements to my app? I have never done anything with Game Center before.