Code for Express requests

The examples in the Express routes/router lessons initially use some helper functions, however, I feel even more confused as I do not know what type of code to write into requests eg GET, POST etc and when to use req.params, req.params.somethingHere, req.query or req.query.somethingHere

eg. I would not know how to even come up with the solution to:

app.put('/currencies/:name/countries', (req, res, next) => {
  //code I do not know how to write here to work correctly