Used iMac
How do I register my used iMac to my Apple account
MacBook Air (2020 or later)
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If you bought it used, please do the following before beginning to use it, otherwise you will have a complete mess on your hands.
What to do before selling or giving away your Mac
After you have executed those steps and re-installed Mac OS you will simply log into enter your Apple ID.
System Preferences - Click Apple ID and follow the on-screen prompts.
Macs are not "registered" to users.