Sorry this part is in the cpanel:
"They have several domains that aren't set up as redirects but rather set up as aliases... (Is there any good reason to do that?)"

@Alien_Mike ... welcome to Webmasterworld!

Having never used alias I can't offer advice, but hang in there, others will have answers!

one reason for alias domains is so that several hostnames can be configured as one virtual host.

in any case, i would suggest redirecting any requests for noncanonical hostnames to the same path at the canonical hostname with a 301 status code.

and welcome to WebmasterWorld [webmasterworld.com], Alien_Mike!

aren't set up as redirects but rather set up as aliases... (Is there any good reason to do that?)

There might be, but the first thing that comes to mind is a bad reason: so you can claim to have sites in multiple countries--example.fr, example.be, example.ch and so on--on the premise that search engines will think more highly of sites in the searcher's home country. Other subforums hereabouts can address the question of whether this is a valid approach. I suspect not, since G### has already figured out what is going on.

the user declared and google selected canonicals are URL's from these alias domains

This is a little obscure. Do you mean that the canonical is different from what you want it to be? Or do you simply mean, under the hood, that the “canonical” domain happens not to be the one set in the server as the “real” domain? If it’s the latter, don't stress about it; nobody but the server administrator knows which one is “real” and which ones are aliased.

I have seen aliases of one of my domains assumed by Google to be the same website as its main domain.

Well that is ridiculous because ISPs used to and still do create and use aliases for their client's home pages, like smith.geocities.com, and all of those sites will have different content and different owners.

Is Google being run by AI? Might as well be!

thanks for the feedback. I have made changes with domains that were purchased and set up as aliases. I think I have that part sorted.

I still see one other problem where pages have been creating with Two or more pages have identical titles. The pages are created on the main domain but duplicated with the subdomains for vps. and mail.

example:
https://main-domain.com/page title
https://vps.main-domain.com/page title
https://mail.main-domain.com/page title

I am a little out of my element here and trying to learn as I sort this out, all I know is it is wreaking havoc on SEO.

thanks for your help once again.

[edited by: not2easy at 7:26 pm (utc) on Jan 22, 2023]
[edit reason] delinked for readability [/edit]

Do you have a redirect set up for each domain so that any request for other forms of the domain go to your preferred domain? A 301 set up in the domain root htaccess deals with that. Our Apache [webmasterworld.com] forum would be where to get help if that is part of the problem.

If you can get to the same content from all the possible URLs that would indicate you are missing the canonical domain redirect.
Examples -

http://example.com
https://example.com
http://www.example.com
https://www.example.com

Is there a reason you want to have copies of these pages in various other places? The duplicate pages where visitors are not browsing is not generally a good thing. Your CMS might need some settings changed.

you should specify the canonical hostname in the substitution string of all external redirects.
you should follow any more specific redirects with a general hostname canonicalization redirect ruleset that will check for any non-canonical hostname requests.