Welcome to the community!

The EA Studio collections are indeed JSON files.

You probably have a folder of individual Expert Advisors  (MQL files) or a  Portfolio Expert (a single MQL file containing multiple trading strategies).

The best way to import and test them in EA Studio is to use the "Validator" tool.
You can open it by clicking the down arrow next to the Generator button.

When you drag-drop your Portfolio Expert or the EA files, switch off the Monte Carlo option (if it is on) and run the Validator.

It will load historical data and will calculate all the imported strategies.

The strategies that pass the validation will be pushed to the Collection.

how can I upload a collection back to the EA Studio?
I´m asking because I´m confused. When I try to upload my collection (99 EA´s from the Ethereum course from Petko at Udemy) to the EA Studio, the program only allows JSON Files to be uploaded.
But the expert advisors are  MQ4 Files.
(I suppose this is a very silly question from a newby, but I don´t find an answer in the WWW, so to ask here is my last chance)

Best,

Connor