Lost all passwords when transfer to new iPhone 12

Do you still have the old phone?

If you check Settings > Passwords are there passwords listed in there?

If you check Settings > [your name] > iCloud, is iCloud Keychain turned on?

If your passwords were stored using iCloud Keychain on your old phone, you might just need to turn that on for the new phone.

If you are just referring to the previous phone signing into an app using Touch/Face ID, that doesn't necessarily mean the passwords were saved anywhere.

Did you have iCloud Keychain enabled on the old phone? If so just enable it on the new phone.

If Keychain was off and you transferred data by backing up to your computer, then restoring the backup, unless Encrypt Backup was enabled in iTunes (or Finder on a Mac with Catalina or later) the passwords were excluded from the backup.