FTP is quite straight forward yet unbeknownst to most, they can be quite finicky with exacting requirements such as equal install of the client software that allows you to communicate with the server. The server’s job is simply to disburse the files according to pre-set file sizes, or packets that are then sent through the internet to be stored into a temporary file on your desktop, to be completed and verified when the process has completed. Aborted downloads or uploads can be continued with many software having their own proprietary protocols that allows this to happen. Others go under the browser, using proprietary protocols that allows it to do file transfers quickly.
First used by scientists who needed to transfer huge files across the rapidly developing internet that was beginning to cap file size limits for more commercial uses. Most of the free mail sites, set-up file caps to limit the size of files that can be sent so they do not eat up too much resources that can be used to serve other clients.