This depends where you put the limit between front end and back end.
Typically modern web applications have three tiers:
- the data layer (unambiguously backend)
- the web server,
- the client (unambiguously frontend).
The web server is where a web app is being hosted. Beyond serving the files and handling client connections it performs some tasks such as sending the data requests to the data layer, getting the response and sending it down the client, security, etc.
Originally React was focused on client-side code. The client would get data from the servers, then would generate an interface.
Increasingly, React is used on the web server layer through server side or hybrid rendering. Server-side rendering means that the powerful web server will get the data it needs, prepare the interface then send it back already made to the client who will just display it. Hybrid rendering means that the server prepares an initial version of the interface, then the client can request additional data and update that interface.