What does launchctl unload actually do? When running launchctl unload some.plist, what is this command translated to specifically, in terms of UNIX commands, etc.

There is nothing in .plist files that specifies any "command line to run when the user specifies unload", so does macOS simply send a kill signal to the process? Or what does it do?
SIGTERM

launchd's unload command sends a SIGTERM signal to the associated job's child processes.

Detached Processes

If a process launched by a launchd job has detached from it's parent process, then unload will not affect that process. This is often the case for daemonised processes.

Original Source Code

You can download and inspect the original launchd source code. launchd has since been rewritten and is proprietary to Apple but the original documents the intended behaviour on unload.