How are values stored and retrieved from sessions?

I am working on a rails backend application that needs to dialogue with an ember client. My question is: once I store a value in the browser session with rails with something like session[:user_id] = user.id, will the ember client be able to read it straight or do I have to go through active model serializer as for any other data? Secondly, do cookies follow the same logic? I am totally new to the session and cookies subject, so I might be stating the obvious...sorry if that is the case! Thank you in advance for your time!