How do I unlock an old iPhone 8 without password?

Please follow one of these two articles based on your case to reset your passcode:

iPhone is disabled: If you forgot your iPhone passcode or your iPhone is disabled - Apple Support

iPhone is unavailable: If you see a iPhone, iPad, or Apple Vision Pro unavailable message or Security Lockout screen - Apple Support

If by password, you mean Passcode, see this --> If you forgot your iPhone passcode or your iPhone is disabled - Apple Support

Are you the original buyer of the iPhone, or did you buy it from someone used? If you bought it used, when the was reset, it locked to the original account used to set the phone up. If you don't know the Apple Account or Password used to set up the phone, you can't use the phone unless you can reach whoever originally set up the phone. If you don't know them or can't reach them, you can't use the phone, unless you have the original purchase receipt, which clearly shows the iPhone serial number on it.

If you bought a used iPhone read this --> https://discussions.apple.com/docs/DOC-250001038