The "Providers", as they are known in OpenFeature, are typically provided by the vendors, or contributed by the community.
For instance, here are is the repo for contributed Providers in Go:
https://github.com/open-feature/go-sdk-contrib/tree/main/pro...
And here is LaunchDarkly's official Java OpenFeature provider:
https://github.com/launchdarkly/openfeature-java-server