(old phone)
Do you still have your passcode?,
(< 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
https://support.apple.com/en-gb/HT212951
Then try this -> 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
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
See also -> If you can't connect or sign in to iCloud
If you can't connect or sign in to iCloud - Apple Support
You can’t merge multiple Apple ID accounts.