What does Uploading files to a Server depends on

i have a confusion when it comes to Uploading files to a web server

What makes it fast or slow?

How much internet does the server use in order to receive the uploaded file, i know it may sound weird but does the server download the uploaded file or just receive it?

And is it the same for FTP and P2P or different from them?

Could anyone clarify this part for me?