(< iOS 15.2) See this Apple article: If you forgot your iPhone passcode
If you forgot your iPhone passcode - Apple Support
(iOS 15.2 and newer) How to reset your iPhone, iPad or iPod touch if you've forgotten your passcode
How to reset your iPhone, iPad or iPod touch if you've forgotten your passcode – Apple Support (UK)
Have you forgotten your Apple ID?, Check your Apple ID via your email box, see this Apple article: If you forgot your Apple ID
If you forgot your Apple ID email address or phone number - Apple Support
OR see -> Having trouble signing in?
https://iforgot.apple.com/password/verify/appleid
If you've forgotten your Apple ID email address or phone number
If you've forgotten your Apple ID email address or phone number – Apple Support (UK)
Have you forgotten your password? -> See this Apple article: If you forgot your Apple ID password
If you forgot your Apple ID password - Apple Support
Then see this Apple article: What to do after you change your Apple ID or password
What to do after you change your Apple ID or password - Apple Support