How can I configure multi-site base URLs where the subdomains are static but the domain name is dynamic?
You could achieve this using Aliases.
In your .env, you could set DEFAULT_DOMAIN=&quot;yourdomain.com&quot; then in your config/general.php you would do:
'aliases' =&gt; [
    'siteAbc' =&gt; 'http://abc.' . getenv('DEFAULT_DOMAIN'),
    'siteXyz' =&gt; 'http://xyz.' . getenv('DEFAULT_DOMAIN')
],

With this in place, you will be able to use @siteAbc and @siteXyz to set your site URL as well as @siteAbc/images for your assets Base URL for example...
Note that all naming convention is up to you, of course...