Parsing a browser game?

Question

reverse engineering

Hello, I can not figure out one thing and I'm looking for advice.
A year ago, I had a desire to disassemble one browser game and make my own pirate server (without donations). Now it has already been covered, but it was called Fairy tail Rpg. It was another browser game from Esprit Games. Therefore, I turned to one forum for advice, and there I was advised to study html and js. There were no problems with this and I already understand them quite well, but I still don’t understand how I can disassemble the game? Knowledge of html did not help me in any way. I've heard about network protocol parsing and stuff, but I don't know where to start. How to disassemble the browser and make your own "pirated" server?
ps I would also like to know about the substitution of packages and their decryption. I heard that the substitution of packages is no longer relevant, but I did not understand for what reason.