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.
Netboot.xyz ?