That’s correct - Server and Data Center integrations are based on Java plugins for the most part, and Atlassian Connect is not present. Instead the Atlassian SDK will help you set up a plugin - https://developer.atlassian.com/server/framework/atlassian-sdk/set-up-the-atlassian-plugin-sdk-and-build-a-project/
You don’t need a license as the CLI for the SDK has commands to start an instance with a time-bombed license for development.
Bitbucket Server APIs are usually listed in our reference docs, however for that particular one, I don’t believe there is any Java API reference doc exposed. The source is in the atlassian-webhooks-plugin repo if that helps. I’ll raise an issue for handling this better.
“If integrations are available for BitBucket Server then where is that documentation?” Not sure I follow this question, sorry. If you’re referring to Connect, then it’s not available. We have a few different specific REST-based APIs that you can integrate with remotely (e.g. adding Build Statuses), but they generally require an access token of some kind. We don’t have a reified “Integration” concept just yet (with authentication and authorization nicely bundled), though it’s something we’ve been talking about.