Grouping of REST APIs

My question is around best practice of aggregating or grouping REST APIs. I have a scenario where there are many different vendors, data sources, etc. and I think grouping REST APIs would make a lot of sense to keep the system maintainable.

I have many scenarios where there will be a single API call triggering many other (similar) API calls. For example for an example entity "client":

1. Front-end calls REST API: PUT .../client


2. What I envision is that the code listening on the above API will then make multiple REST PUT calls to let's say vendorA/client, vendorB/client, vendorC/client, internalSystemA/client, internalSystemB/client, etc.

What is the best practice to group REST APIs in such a scenario? Possibly by vendor, functional, domain? How would the URL look like?