Mail Alias address appearing on one Mac, but not on other devices.

Hello everyone,

I have a primary apple ID email address and two aliases (if that is the correct term for the object I am describing ... two additional email addresses ending in @iCloud.com).

I can view all three on my new Apple Mac M1 Air (thanks to a genius who sorted it out month ago when I first bought it.).

I can view two (the core ID address and one alias) on my older MacBook Pro and iPhone 12.

I cannot find a way of displaying the 2nd alias on the old Mac or my Phone. I cannot recall the password for the alias and when I attempt to use password help, it does not send a verification code to my other devices. Password recovery appears to do NOTHING. (And I've requested it multiple times.)

Once upon a time, Macs were intuitive and easy to use, even for techno-fools such as myself. Now? It's getting almost as bad as Microslop [sic].

Any help would be greatly appreciated.

Many thanks