Basic HA question: Who decided if a new feature should be included as a core integration versus leaving it be offered by the community (HACS)?
For example, if a company offering a new product wanted good HA integration, is there a governing body to allow it into the core project? I assume there is a process of some type.
I think you submit it, answer the questions, fix all the things that need to happen to be merged.
I would suggest asking in the discussion area to get a feel of what is needed before you start working on it.
Also there is a Developer section on the HA Discord where you can read and get questions answered.
You can just create a PR with the new integration.
For guidelines on adding a new integration see Contributing an integration to core | Home Assistant Developer Docs
I would suggest to look at PRs of recently added integrations to get a feel for how it usally goes.