Dealing with a messy prescriptive monolith. How do you survive this? [D]

Dealing with a messy prescriptive monolith. How do you survive this?

Months ago, I got my first maintenance project. Before this, I had only built new solutions from scratch and maintained my own code. But maintaining someone else's system feels completely different.

It’s a prescriptive recommendation system that uses XGBoost models and Differential Evolution for optimization. The problem is that everything is in a single repository: raw data ingestion, transformations, model training, reporting, the optimization engine, post-processing, and MUCH more. The only thing outside the repo is the frontend website. To me, it looks like a massive, super complicated monolith.

After almost 3 months, I still find new "patches" (quick fixes) every single day. Every time I do, I have to re-learn how the system works. The documentation is very generic and a total mess; it mixes the original design with patches from the two maintenance teams that came before me. I’ve checked some of the docs, but definitely not all of them, because there are about 50 long markdown files.

Have you ever dealt with a prescriptive system like this? How do you survive? Honestly, I’m debating whether to just quit or keep patching the code however I can until the project ends-even though I know that’s not the right way to do things.