I only learned a couple years ago that some people "tune" their cars (teens mostly, I assume, or that sort) by simply doing some sort of chip upgrade. No idea how any of that works, but I'm sure it's a deep rabbithole

Yup, that's it. In the 90s it was called chip tuning colloquially. Usually switching to eeproms over the stock proms and continually swapping rom chips out after modifying lookup data. The only difference now, is that fpgas and sram got fast and cheap enough we could socket active devices directly onto the bus instead. Same old 90s vehicles, new tech look.