Hi, thats pretty broad!
You may want to start here:
https://learn.microsoft.com/en-us/azure/active-directory/hybrid/connect/how-to-connect-sync-whatis
To see what is being possibly excluded, check the filtering options:
Sync is every 30 minutes by default or can be forced manually:
Sync is one-way AD> Azure however there are writeback to on-prem AD options for groups, devices attributes and passwords