Remote Desktop Connection is described as 'Technology that allows you to sit at a computer (sometimes called the client computer) and connect to a remote computer (sometimes called the host computer) in a different location' and is a very popular Remote Desktop tool in the network & admin category. There are more than 100 alternatives to Remote Desktop Connection for a variety of platforms, including Windows, Mac, Linux, Android and iPhone apps. The best Remote Desktop Connection alternative is RustDesk, which is both free and Open Source. Other great apps like Remote Desktop Connection are AnyDesk, Remmina, Chrome Remote Desktop and mRemoteNG.
Xpra is a X server and lets you run remote programs that have their screen output on your local computer. That is not the same as actually copying a remote screen output to a local computer, which is what remote desktop does. It might have some advantages in some cases and due to the technical nature has a lot of limitations. That's why it is not an alternative to remote desktop.