The Yii2 quick start example for REST gave those endpoints as example:
GET /users: list all users page by page;
HEAD /users: show the overview information of user listing;
POST /users: create a new user;
GET /users/123: return the details of the user 123;
HEAD /users/123: show the overview information of user 123;
PATCH /users/123 and PUT /users/123: update the user 123;
DELETE /users/123: delete the user 123;
OPTIONS /users: show the supported verbs regarding endpoint /users;
OPTIONS /users/123: show the supported verbs regarding endpoint /users/123
Well that is REST. It doesn't matter which tool or framework to use. You can rebuild the same using Ruby, C or NodeJS and steal being the same app. Just have it in seperate folder or server and call it backend or server while it retreives data from DB or whatever else to answer the previous URI's.
The frontend in this case is the other app built with whatever language stored in a different folder or server that when it does a request to GET /users will expect a json response. when it does some edits on user 123 and send it back to server within PUT /users/123 it will expect the server to responde with a 200 response to know that it has been saved to database or a 422 error for example if validation fails.
My advice is to work with both tools as separate things. I don't recommand changing Yii's default rooting. The commun point between both should be the standards and concepts or architecture you did decide to implement or design for your HTTP requests.
More answers and a valid structure for Yii and angular may be found in the following:
What exactly is RESTful programming?
Yii2 + AngularJS in a single application - how?
