Grouping and nesting 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.

Like this:

                                                            +-------------------+              
+--------+                   PUT     +-----------+  CALL    | Vendor A API      |              
|        |                 +-------> | Client    +--------> |                   |              
|        |                 |         +-----------+          +-------------------+              
|        |                 |                                                                   
| Front  | PUT    +--------++  PUT   +-----------+  INSERT  +-------------------+              
| End    +------> | Client  +------> | Client    +--------> | Internal System   |              
|        |        +--------++        +-----------+          +-------------------+              
|        |                 |                                                                   
|        |                 |         +-----------+  CALL    +-------------------+              
|        |                 +-------> | Client    +--------> | Vendor B API      |              
+--------+                   PUT     +-----------+          |                   |              
                                                            +-------------------+             
                 +                                 +                                           
                 +---------------------------------+                                           
                         Internal REST APIs                                                    

What is the best practice to group and nest REST APIs in such a scenario? Is it a good idea to group them by vendor or should they be grouped functional or by business entity? How would the URL look like?