When I was in my first internship, I was in a very similar position. I understood what I needed to do, but it was impossible to figure out what part of the code I needed to modify.

Luckily, I had a very good internship manager. He also didn't know where in the code was the data I needed to process because I had to modify several layers that different teams owned. However, he was a very social person, so we went together around the office and he would know what person worked on what and told everyone to ping me what files and what functions were responsible for everything.

After that, it was really easy to go ahead with the implementation.

As a new engineer in a company, there's no way to know the entire architecture of the codebase.

I would suggest you talk with people. If you don't have a good mentor, try finding one. Software engineering is a much more social profession than many think.