Here's the scenario:
I have one website with the distributed domain www.mydomain.com, but I've got multiple domains like
mydomain.com
my-domain.com
www.my-domain.com
mydomain.org
www.mydomain.org
my-domain.org
www.my-domain.org
From a SEO standpoint, the expected behaviour when opening my-domain.com would be a redirect to www.mydomain.com, correct?
Is this something that TYPO3 Siteconfiguration can handle? I can't seem to get the desired result there.
Usually those things are handled on webserver level, directly in the server configuration or in a .htaccess file.
Nevertheless You can try to configure it with the extension "redirects" (typo3/cms-redirects) which shouldn't be a problem.
Consider if you want to forward path and parameters too.
Another options that is often chosen to log page calls over a special link is to create pages with redirects. The origin page call is logged then and the target page after the redirect too.
TYPO3 offers you a specific module for redirects, so the solution would be a combination of both, Site configuration and redirect entries.
On the other hand, maybe it would make more sense to move that redirect behaviour to your webserver instead of having TYPO3 handle it.
And IMHO it would be better to redirect from www.xyz.tld to xyz.tld, unless there are other use cases of those domains than providing a website for the www.