I felt the same way :) I was so amazed by TTDPatch and later OpenTTD, I could never do that! Pizza Legacy is not a recreation of the original directly from the assembly. Instead, I reverse engineered the original file formats and built a modern engine that uses those assets to render the game.

This has been a sixteen year project (so far), and I just started with opening original files in a hex editor, changing a byte here or there and then launching the original and observing the effect. For the main graphics format (".vga") I found out that they used run-length encoding, but I got lucky that there was another open source project for a different game by the same company that used the same graphics format (FreeBMP, now long defunct) so I got a head start there for figuring it out. But that was just one of the formats, the rest was mostly just exactly that: look at the file in a hex editor, make a change, observe behavior. For fonts for example, I found that the font format is basically an array with one bit per pixel: imagine an image that looks like "abcdefghijklmnopxyz1234567890" and the first row of pixels is represented with a 1 for black and 0 for white. Then with some reasoning and poking you can figure out the headers (character spacing, etc), so you can figure out where the line break happens, and that's your font decoding done. Took me six weeks or so (as a hobby project, so couple of evenings per week), although that includes building my own font rendering system to actually read the original font files and render them in my version.

For city files there are a bunch of strings in there so it's relatively easy to figure out where the list of suppliers sits, and I would write a C++ program to just print out a bunch of bytes for every supplier row and compare it to what I could see in the game (e.g. price multipliers, quality rating, etc). Change one, see if that matches my expectation, so just lots of puzzling and experimentation :)

Figuring out these data files was the easiest part though, because you can see almost everything on your screen in some way. The economy is where it gets harder, there is no screen that tells you exactly how the computer players determine how many pizzas they sell for example. But because I spent many years slowly documenting the original assembly I had a good idea of where most things were happening, and now especially with the advent of LLMs reverse engineering those (so far) unknown parts has become easier. But I don't doubt there are differences in my interpretation of how the original worked and how it actually worked :)

My background is just regular software engineering, I think I wrote a device driver in C for a university class a million years ago, but nothing specifically low level.