I am reading this official Microsoft documentation SharePoint classic and modern experiences, which mentioned that there is not a 1:1 map between site collections and hub sites, and we can not move a site collection to a hub site:-
There is not a 1:1 mapping between site collections and hub sites, so you can't automatically move a site collection to a hub site.
But on the other hand, there is another Microsoft official documentation, which mention how we can register a site collection as a hub site Register-SPOHubSite. so i got confused..So if we have a site collection (modern or classic) which contain data and lists, can we define it as a hub site?, that other site collections can link to? as the info in the above 2 links is confusing, unless i did not understand it well.