Ruby on Rails + small question

Hello, I have a question about Ruby on Rails : I want to create a web app, but I don't know if I should decouple the backend and React frontend (using Rails in api mode). I feel like I lose the advantages of ruby on rails if I do that, but obviously it scales better, and maybe that's what companies do most often. What do you think ? Also, what are the supervision times on the timetable, and how to use them, is there an office/zoom to go to to ask questions ? Thank you.