Most intern/juniors developers have a lot of energy and if you let them they would rewrite your application every day.
However, this is not the best way to be productive when learning a new codebase.
The best way to learn a new codebase is to imitate the current code patterns.
I call this smart copy and paste. Instead of building the new feature using all new code, you should find what is the closest code to what you are trying to do and just start from it.
If you copy a working code it has more chances of keep working, even though it is harder to read and understand at first, I bet it passed the test of time and it has almost all edge cases covered.
I always had this doubt, if copying the way some codes were written in my job was good practice or bad practice.