a cryptic hunt website which involves authentication, a DB of questions and answers for the hunt, and security to prevent any compromise of the answers, a dashboard for a competition which involves authentication and forms. Is React sufficient enough to make this or I will require Express and MongoDB as well?
You will require a server component of some sort and a database (doesn’t necessarily have to be Express and Mongo). React is “just” a client-side library for building UIs.
As with everything, it depends. If your goal is to build the app you describe, you’ll definitely need a server based component, so why not Node?
My advice would be to get a handle on Node, get a feel for Mongo DB (here’s a course that teaches both) and learn how they work together (maybe build a small app, or a subset of the original app you want to build).
Then add a view library into the mix (React is fine, I guess, but you could also take Vue, for example), Build a React app and then hook it up to your Node API, googling problems and asking for help as you need it.