If you stil have / know that 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
See -> Change your Apple ID password -> Change 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 the following Apple articles:
If you can't connect or sign in to iCloud -> If you can't connect or sign in to iCloud - Apple Support
Contact Apple for help with Apple ID account security
Contact Apple for help with Apple ID account security - Apple Support
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Your Apple ID is the personal account you use with all Apple products and services. It includes the email address and password you use to sign in, as well as the contact, payment, and security details that you use with Apple.
You can’t merge multiple Apple ID accounts.
See -> Sign in with your Apple ID -> Sign in with your Apple ID - Apple Support