Strictly speaking, they’re leveraging free users to increase the number of domains they have under their DNS service. This gives them a larger end-user reach, as it in turn makes ISPs hit their DNS servers more frequently. The increased usage better positions them to lead peering agreement discussions with ISPs. More peering agreements leads to overall cheaper bandwidth for their CDN and faster responses, which they can use as a selling point for their enterprise clients. The benefits are pretty universal, so is actually a good thing for everyone all around… that is unless you’re trying to become a competitor and get your own peering agreement setup, as it’d be quite a bit harder for you to acquire customers at the same scale/pace.
I think it would be a good idea to take a step back and ask what is it that you’re trying to achieve.
Userbase, the service linked, is a backend as a service platform that offers you authentication and basic database that you can access via their api. You’d then code your own front end web app to interact with their service and store data there. You pay only per storage used by their storage tiers, which are frankly fairly fair priced. If that is something you’d need, that’s a good idea, but you’d be coding the front end yourself.
If you’re only looking for authentication with OAuth, and then coding your own API backend, then something like Authentik would be a nice self hosted authentication provider. Others that commonly gets mentioned but I’ve got limited/no experience with worlds new keycloak, or fusionauth. Managed services here would be your Auth0, Okta, etc.
If you’ve got a specific use case in mind, then it may be a good idea to say what service you’re thinking about, and the community may be able to suggest prebuilt solutions that good better and require less lift.