The domains do seem to be yours, since both services let you transfer the domain away:

https://support.wix.com/en/article/transferring-your-wix-dom...

https://support.squarespace.com/hc/en-us/articles/205812338-...

As far as I know, their SEO is fine, though they'll usually do worse in things like page speed than a site built from the ground up. I don't recall any sites my previous employers built with said services struggling to rank in Google.

As for whether they'd be recommended? Well to be honest, I'd say only for small companies and individuals who need the most basic of websites and don't fancy paying very much for it. For a client in that situation, you may as well just throw together a quick site on one of these services, change a few images and colours and call it a day. At least then they won't keep coming to you for web hosting help or updates.