Congrats on dipping your toes into networking! Don’t let it suck you in too much or you’ll end up with a career change.

Plenty of resources out there to learn from, just pick a project and try to implement it. Or just play around with netcat (just ‘nc’ nowadays).

Look into the TCP/IP stack (or the OSI model, both cover the core concepts) for an overview of how applications talk to each other. This will also help you understand how LocalSend probably works (my guess is broadcasting to your network and seeing what devices are listening on a certain port. Some LAN-enabled games work like this, others aren’t as magical and ask you to provide an address and a port).