[chupasaurus]

> How does it work?

KDE framework has KIO library which provides network-transparent file access (that's why Konqueror works a file manager and a browser simultaneously). IIRC the author of KDE Connect was tinkering over sending notifications to a phone app and found out you can make much more with it.

> How is the connection between devices set up?

The server part uses 1716/UDP for broadcasting it's presence and 1716-1764/TCP to communicate with clients. The client had to be paired with server (like in Bluetooth), the communication is done via TLS, file browsing works over SFTP.

> How does it control apps and receive notifications?

Everything is done via plugins, both sides choose which ones are enabled and some have their own control schemes (i.e. whitelist of apps to sync notifications from phone).