This kinda thing is so rough, and I can really relate to the questioning of your own sanity part when everyone around you is insisting something works in a particular way that is just factually incorrect.
I got hired into a pretty old small technology company that has over a decade of tech debt, and "how the whole system works" is different depending on which engineer you're talking to. You have to do a context shift to a different engineering reality just to do basic improvements to the system. Lots and lots of built up confusion over years of incremental changes, some of which under pressure no doubt, some well intentioned half-refactors, some almost dead code...imagine your well established corporate morass and then give it a shoestring budget.
It's scrappy in its own way, but the threads where people advocate "don't worry about the tech debt, if the company succeeds they will have the budget to fix it" don't account for the middle ground of not having huge success but having enough success to continue indefinitely. I guess that could mean you could fix the problem over longer time spans, but people do t stay at orgs like this long enough for that to happen, because the job of fixing it is no fun and you can't just throw huge amounts of money at the problem.
