There are several protocols to connect to your PC by remote The most used are VNC and RDP: the first is open source but less secure, the latter is more secure but proprietary (so i discourage you to use it).
The choice mostly depend on your needs and the kind of the net you are: if you need to reach your PC from insecure network you also need to provide a more secure tunnel such as SSH, OpenVPN or WireGuard.
A good simple choice may be VNC(server)+OpenVPN(server) on PC and VNC(client)+OpenVPN(client) on the smartphone or remote PC. If you use this in a secure local network you may try VNC only (but if someone can access to your home wi-fi network he may cause a real security issues).
If you only need to connect to your PC from your smartphone and vice versa you may try KDEConnect as suggested by @astragalo.