Passwords.

Not sure if this is a MAc or a Google problem 10 or so years ago I changed from googlemail to gmail with all the attendant problems. I have just transferred all my data from an iMac running 10.11.6 to a Mac running OS 10.15.4. A lot of apps where my gmail address is my identifier are n ow looking for googlemail. One example is my iCloud account. I have tried using systems preferences but seem unable to make the required change. There was a different password for googlemail to the gmail accounts which is now causing some conflicts.