Should I add my integration to HA Core or python wheels?

I’m currently working on a custom integration for a device. The integration is now in a working state but I’m a but unsure whether to make a PR for the HA core repository or if I should focus on the Custom Integration Wheels repo instead?

Are there any official guidelines on what kind of integrations should go where?

Have you already had a look at the developer site? That has some guidelines around developing integrations.

I have read the developer docs and haven’t found the answer to my question.

No worries. So, I don’t think there are official guidelines telling you what is supposed to go where, but in general, the code quality expectations for a new integration are quite high (developer checklist), you would need unit tests, documentation, etc.

If you just want to keep your integration as a custom integration for now, then I would still recommend to follow the best practices mentioned in the developer docs as much as possible so that you have the option to migrate it into a core integration later on. Many people do publish their custom integration via HACS.

That custom integration wheels repository really just contains metadata about custom integrations and there is some explanation in this blog post.