Create a Json view of the user, with the format of the user/login response
When you query /user/login it defaults to currently logged-in user.

I suppose you need to create a view with a "REST export" display. The view defaults to "Entity" row style, change it to "Fields".



In "Fields" settings, you can rename the labels that are output in "Alias" fields:



In this "REST Export" view you will have set a path. i.e. /rest/people

Querying /rest/people?_format=json should output something like this:

[
  {
    "user_name": "admin",
    "user_id": "1",
    "user_customtext": "Custom text"
  },
  ...
]


You can use the Global - Custom text field to concat multiple fields as tokens, then hide those fields from being displayed: