Java Reflection with Client Written in Java

this might be a little too specific but I'd figure I'd ask anyway.

I'm aware of how to use reflection in a pretty general sense. I'm also aware of people being able to use reflection to get classes from a client and then pull values from them.

My goal is to be able to pull classes and player data from the Old School Runescape client. Most people do this to make bots, others to make clients to play in. My goal is to just learn about it and possibly make a client to play in, but mostly just curiosity and to learn.

I've tried looking for results on how people use reflection to create bots or interact with the runescape client in any way, but all I have found have not been helpful.

The biggest challenge I face (and maybe this is super simple), is how to even grab the classes from the client. How would one go about even just loading a window via Java, and being able to play the game inside that window? This is the least of my problems but I guess it's a starting point. (I haven't looked this up since it seems like it will probably be trivial).

In all, I'm really just looking for a pretty basic way to grab the classes from the client. I believe that if I got this far I could then start getting other information, but since I don't even have this basic ability down, I feel as though I'm completely lost and can't even start!

I've seen reflection code that loads a java class, but I don't even know where I would find the OSRS java class to use, or how I would hook up my program to read this class from the existing OSRS client.

Appreciate any help.