That happens on my RSS reader, I haven’t looked into it too deeply but I’m assuming he’s using JavaScript to populate the entries and are thus not being populated on none dowser clients

I don’t know about local send specifically, but KDE Connect will do that. And if you have an FTP client on your phone, then yes you can easily spin up an FTP server on your local network and transfer files that way

Spotube is an android app that provides a frontend to Spotify and allows you to download songs you listen to to your device. Im guessing you could sync those files to your server and store them in a different system.