I think it is a dyndns service.
Personally i use https://www.ddnss.de . You can use different domain suffixes.
@jamess7995
If you have an own domain you can map e.g. cloud.my-private-domain.tld with CNAME to the dyndns name and then configure nextcloud and Lets Encrypt for your sudomain name. That looks better than the dyndns name.