React Router is there to allow you to split up your app UI into sections, it is purely a UI abstraction. Express router is there to allow you to define endpoints for different resources.
So as an example, say your app is just a form with multiple pages. In that form you have some data that needs to be validated/saved/etc as a user goes through it - say the user email and address needs to be checked.
In express, you have a single route for the HTML index page. That route can set up the page with the initial props to set up the React form.
In react you would have multiple routes, but they are purely a UI thing; it’s still the same page. You can hook into them to run logic though, like fetching data before the app updates to the next page. So yes it doesn’t allow props to be passed, but the component update functions are there to allow you to update state, and you pull from the backend there.
In express you would also have a set of routes that just return JSON. So when a user enters their email then clicks a button to move to the next page of the form, you could hit the relevant Express route that validates the data and returns a blob of JSON you can use to update the state of the React app. Same with the address. Then when the user hits submit, it hits that submit route in the Express app, which saves to the DB and returns something that says everything’s ok or its failed.