I'll answer this question from a perspective of a regular web react app to give the idea, you can collect my ideas into your react native apps. even though I'm not a react native dev.
@ the login page, your app will send a POST request to the server, in which the HTTP service will respond back with the user document as a JSON object, in this JSON object, there are many fields, in one of those fields there must be a field with the name of something like "accountType" or "user_role".
You can use that field and store it globally somewhere in your react/react native application.
Then, you will create two separate folder structures, with the names of the roles, ex: "seller/", "buyer/", "admin/".
inside these folders, you'll write the UI for based on every user role from the ground up.
during that, you'll find many components that can be reused, these components should go in a directory outside the role folder.
The reason I think it is the best approach for UIs that has too many differences based on their user roles is that I used to think that these roles UIs must be separated into different react applications,
for example, a react application for admin, a react application for user.
however the problem with this architecture is that you'll have a third react app just for the login page.
and in this third app, when a user logs in through it, you'll have no way to pass the state object from this react app to the other react apps.
you could pass it through the web browser local storage or something like that, but that's too much messy and not desired,
also there are limitations on how much space the local storage key and value can stores in the browsers.
you could use the second approach, but you should then send a GET request to the server through the user
or admin
react application to get the user document (or data), so you can render the content for that. which is a bit better than the approach of passing to local storage.
For recat on the web
I would only recommend the second approach if you don't want the regular users to know the api keys, URIs URLs or like that of the admin page.
some stuff must be private and should not be served to the client, even if the api itself has restrictions. the front end should also restrict some stuff and not to show some data in its source code sometimes.