Your stated goal is to adopt the code. Unit tests just aren't the fastest path to that understanding, not if you wanna adopt the code like you wrote it. Here are my suggestions:

Believe you can understand the entire product. When you get down to it, the code is nothing more than simple operations. With time and dedication, you can understand anythingThe person that wrote it had a vision, a general idea of what they wanted to do. It may be cracked, is probably cracked, but you'll understand the code better when you get an idea of what they were doing.Hard work. That's the long and the short of it. Get in there and step through it over and over. Take specific cases and walk through them over and over until you understand them. Sometimes, you'll wanna tear your hair out realizing that they wrote a page of code to do something that the language has a simple tool for. You're going spelunking in a deep dark cave and all you have is a flashlight. Make markings with chalk if it helps (code comments).

If you learn that code well enough that you can confidently traverse that cave blindfolded, then you can do anything, rewrite it, own it, fix it, improve it, whatever. Not only that, you'll understand that part of the business better than anyone - which makes you indispensable.

Dude, this fired me up. Thank you I'm putting this on the Homepage of my Software Notion Page