No, but there are both open source libraries, and complete authorisation servers. If you trust Okta, you can trust these, and embed them in your infrastructure. Auth only gets super hard if you try to be smart and deviate from best practices.
Keycloak, for one: https://www.keycloak.org
I've also heard good things about Hydra: https://www.ory.sh/hydra/
Depending on your needs, an SSO proxy like Vouch works well for internal services: https://github.com/vouch/vouch-proxy
There's also an abundance of libraries for different programming languages to implement authentication or authorisation, lots of them battle-tested by thousands of services. It's not like providing secure services is impossible without Okta.