Glad to hear you enjoy it!
That sounds a lot like memory scanning for an unknown initial value, then looking for a decreased value etc. This process will (usually) allow you to find the address in memory that stores the ammo count. From there on, by setting a breakpoint on it, you can find the code that modifies it and for instance NOP it so that it no longer does anything. Cheat Engine is a great tool for that and all you really need initially. A more powerful debugger would be x64dbg (which is free). For static analysis you could use Ghidra (free) or IDA Pro (very expensive), but I'd recommend starting out with just Cheat Engine initially.
Once you know what to patch, you could just write a small C++ library that scans the game's memory for the location to patch and then applies the patch. I use Visual Studio for that. Game archives vary wildly and you usually have to rely on a third party tool (like OpenIV) or try to document it yourself, usually by using a Hex Editor and studying the code to parse the files. That's a different story, though. For the hitmarker no files are touched, but only game code (compiled code) is modified to skip a certain portion used to render the hitmarker (essentially removing an if-block).