How do I login on a new phone with my previous iCloud account that I don’t remember

You mentioned that didn't remembered iCloud account that is not signed in the device see the link https://iforgot.apple.com/appleid#!&section=appleid

You have to use a web browser like Mac and enter the first and last name , the Apple ID address and the characters displayed on the screen , click on continue .

Entering correct credentials will verify the account a green right check mark will appear .

Sign out your existing primary Apple ID from the new iPhone Sign out of iCloud on your iPhone, iPad, iPod touch, Apple TV, or Mac - Apple Support and sign in with old Apple ID and password Sign in with your Apple ID - Apple Support

The photos will sync from iCloud , when iCloud photo library is set Set up and use iCloud Photos - Apple Support also other apple inbuilt applications box has to be turned as on to sync from iCloud Sign in to iCloud on all your devices - Apple Support