You will most likely find KDE Connect as a package in your distribution's repos. If you use desktop environment other than KDE's Plasma, you might also want to install indicator-kdeconnect, that provides a system tray as a GUI for other desktops.

For Ubuntu users, the package in the repos might be outdated and it's recommended to use this PPA instead.

The app for Android can be found in both the Google Play Store and the free store F-Droid.

If you are a Firefox user, this cool extension might be of your interest.