Activation Lock & ownership
Activation Lock is Apple's server-side ownership lock. For a verified owner there are two real routes: the previous owner, or Apple Support with proof of purchase. Everything else is a workaround with limits, or a scam.
Activation Lock: the official route
The full explanation, start to finish.
Apple Support Activation Lock removal: how to request it
Documents, process, and what happens if it is declined.
Forgot your passcode? The official fix
Erase and restore from backup.
iPhone Unavailable: what it means
Too many wrong passcodes, and the way out.
Lost or stolen iPhone: what to do first
Lost Mode first, then carrier, then erase.