I understand the fact that all of them are “complete” web framework solutions in their own right, but what are the differences that they have, meaning how do they differ from each other in the solutions they offer, and their architecture; when it comes to both front-end and back-end.
Disclaimer: I haven't used any of these specific stacks, so I may not be exactly correct with any of these, but I'll try and give the ideas behind them to the best of my knowledge. ☺️
MEAN: The main thing to note here, especially vs MERN is that Angular aims to be an all-in-one solution to your front-end. It has built in services to handle things like ajax calls, parsing different kinds of data, and has a pretty specific structure it expects you to follow (MVVM which means controllers, models, views, etc).
MERN: React is obviously the main difference again here, and unlike Angular, React is only intended to be a view layer. This means that it helps you design your views and their functionality, but doesn't necessarily expect you to follow any sort of patterns like Angular does. So, to my knowledge, (I've only played a bit with React doing basic things) you need to handle other basic things (like ajax again) yourself or find another agnostic solution to do it for you.
Meteor: Meteor is fairly different from the other two in that it's isomorphic. This means that your client side and server side share code. I haven't used an isomorphic framework myself, but from my understanding you can do things like refer to collections that would be on your server from within your client, and Meteor, or whatever other isomorphic framework you use, will automatically have this on the client for use on the page (I'm not sure if this is pre-renders server side or if it's handled as an ajax call, but I presume the first). Since the server and client are essentially one here, everything is rendered server side first. Of course you can do this with React as well (in fact default choice for Meteor is React for the front end now.) but I believe there are some subtle differences between server side rendering and isomorphic applications, though since I have basically no experience with either of those I'll leave that for someone else who's better equipped to answer that question.
Hope this kind of explanation is what you were looking for. 🙂