I hve just implemented this after we moved away from SuperTokens. My takeaway is that its easier than you'd think (there are libraries that do interaction with the SSO provider for you) and you can fine tune it to your liking (for example, more involved account linking).

If you're starting out though, probably go for a SaaS in the beginning. But be sure to have monitoring for pricing and an option to close account creation, these things can become expensive fast.