Reformatting Mac... what are some files that are automatically saved?
When you reformat a Mac, nothing is really left over or "automatically saved". After formatting, every file on that volume is gone (in terms of being visible through ordinary applications - be aware that it is possible in some cases to perform recovery of the formatted drive, so it is not a safe way to ensure data is permanently deleted).

When you use the macOS Recovery system to format and re-install macOS, you're offered multiple ways of adding your data back to the formatted drive. This is mainly in the form of (a) restoring from Time Machine backup, (b) migrating data from another computer or (c) adding an iCloud account that could then sync over data from iCloud Drive (not in your case), and various other iCloud services such as Photos, Contacts, Mail, Keychain, etc.