You have touched on an important aspect of XM Cloud that separates it from previous versions of Sitecore. XM Cloud is not the place to be serving custom APIs from, and not the place to be doing the kind of coordination that you are talking about.
The lead architect on the XM Cloud project has said:
"You should not be using XM Cloud as compute."
We can extrapolate that statement in numerous ways, one way being that the intended purpose of XM Cloud is to serve as a CMS -- not as a place where you host custom APIs.
What you are describing can be done in the "head" of the application (Next.js); specifically, Next.js API routes allow you to build a public API that runs on the server side. So if your API needs to make calls to other APIs, or perform sensitive operations, or run GraphQL queries against your content (perhaps via Experience Edge), you can do and probably should do that in the head.
My advice would be to get familiar with JSS+GraphQL. There are available features that may meet some of your requirements. For example, you can modify GraphQL schemas and hook external APIs onto them:
Lastly, if you can be more specific about what kind of APIs you want to build, that context may be helpful for other users in this community.