What Does iTunes Backup actually backup for you to Restore?

I've regularly manually backed up my devices (while they're not signed into my AppleID) to iTunes for years, and have only had to restore my contents a few times, but upon doing so, have noticed that some things didn't come over with the restore - such as links saved to the home screen like you would a desktop, some Safari Bookmarks, open tabs, and other things as well like some select (but not all) Reminders and Notes, etc.

I have a situation where I need to perform an iTunes (device) Restore, but don't want to lose anything. What all is an iTunes backup supposed to actually back up, and why might some things not get restored with the rest? All devices have sufficient free space.

Is there anything I could do to make sure I get EVERYTHING without using iCloud backup services?