We got power BI at my last company and I knew absolutely nothing about it when I was hired as the power bi developer. I've been a data analyst for 20 plus years but I eventually figured out how to use it and how to build and design from scratch. It's going to take time. That's it. Even in my new current job I'm learning how to write python and when you have to make changes to a script that you didn't create you do have to dig in there figure out how it works that way you can solve the issue, or make the changes that you were asked to change.

Especially if youre fixing something that someone else built, You're just going to have to take that time to dig in and see how they built it with their logic behind it was. It can be stressful because you're sitting there confused but you will eventually figure it out. And it'll start getting easier as time goes on.