i plan to learn th MERN stack (i'm still learning JS though) and i want to know if it is possible to create a android and ios app with this stack, and if the process is easy and how the app will perform in comparison to a native app.
Use react-native-web. It's from Twitter. They use it for their apps.
The only parts that matter from MERN for app development is the React/Node part (assuming you use React Native)
MEN is in relation to your server side which can literally be ANY STACK.
I would caution trying to learn so many things at once, personally, and I think it depends entirely on what the app needs to do.
My own app (https://thoughtreps.com) is PERN (postgres instead of mongo) and I am currently writing the iOS app in native swift.
Happy to offer more information if you'd like to tell me what sort of features you'd need for your app!
Yes, but with limitations.
A server, yes; if your app needs an online server, that could be done in node, express, and mongo.
The actual app, kinda; you could use react native, which is close to regular react, although you may hit problems as react native doesn't have access to all the native features that you could with the native languages like Java or Swift.
Generally the "build a native app with your web stack" options come with limitations and caveats.