How is the process of uploading a file to the server?

Question

Ruby on Rails

I'm interested in general terms. That is, if I understand correctly, then the file selected in the browser is sent by the browser streaming, where the stream receives nginx (for example), passing it on to the Rails application, but what's inside is already a question.
I would like to make a loader that shows the process of downloading a file, but I don’t understand where to start.