1 - how should the learning ranking be (Mongo db,Node.js,Express)
2 - is it only available for the web or is it also available for mobile?
3 - What do they do
4 - I want to use it in my mobile application on Flutter, but do I need to learn javascript for this, or do these programs also work with the dart language?
I will try to answer your questions briefly.
1- For me you should learn the basics of js language first then you can learn nodejs side by side with express (Express js is the server of nodejs framework) then you can learn Mongo.
2- Nodejs is just backend (server) soo you can use it with any client (frontend) you want (web - mobile - desktop ...)
3- As I said before nodejs is your backend server and mongo is the database soo you can send make your site (in your case you mobile app) dynamic you can learn more from https://bootcamp.berkeley.edu/resources/coding/learn-node-js/how-mongodb-and-node-js-work-together/ .
4- I have answered this qst in point 1 but in Addition I want to say that you can create your server side with flutter framework too.