From what I can gather, one doesn't need to purchase Salesforce Experience Cloud licenses in order to create a Experience Cloud site (aka Community Site) provided that you don't require certain templates that are only included in an Experience Cloud, such as the Microsite Template and that you don't require contacts/users to login.
Is that correct?
The semantics are quite confusing because the name 'Community Sites' would be indicative that this feature is exclusively available for Experience Cloud but that doesn't seem to be true because an org only on the Sales Cloud, as an example, could create a Community Site (up to 100, in fact).
Best Answer
Correct, you get up to 100 community sites included with most editions (I say "most" as I'm not up to date with all the latest editions SF have come out with, but Enterprise and up I know for sure).
As you say, if you want things like logged in users, etc, then these are paid extras.