When it comes to "dominating" the search results, most SEOs define that as taking up as much of the search results on page one as possible. Sometimes that means on one domain, sometimes subdomains and sometimes different domains completely. Jonas Sickler shared on Twitter indented Google results with subdomains and then Ahmed Alaa showed four indented results.
It was for a query on [usps change of address] - here is a screenshot:
You can see that usps.com has four results at the top, when generally Google gives two listings from the same domain - not always, but generally it is two. Do you think that since there are only two www.usps.com listings and then a moversguide.usps.com and a faq.usps.com listing that Google is allowing more? I mean, clearly Google is grouping these results together in one bigger listing with four results. Do you think it would show all four if they were all on the WWW?
Google began indenting results a year ago July (2021). In 2019, we saw other forms of grouping results.
In any event, I found the thread interesting - not in that Google will show subdomains in a group under the www of the domain but maybe that Google might show more if it is using a subdomain for some listings that match the query. Or maybe the USPS should try to consolidate some of this content - maybe that is the take away? :)
Forum discussion at Twitter.