Here's another point of view: don't read, nor attempt to understand, and certainly don't change what is not immediately relevant to the first bug you are assign

Here's another point of view: don't read, nor attempt to understand, and certainly don't change what is not immediately relevant to the first bug you are assigned. Use the bug to guide your learning. Write a test for your bug... no testing framework? Add the first unit test, for your bug. Each task you do will guide you into the relevant part of the code. If you never venture into e.g. utils/common/math/sin.cpp that either means it is correct code you should not change, or that it is unused. In either case, it is irrelevant to your job. Unfortunately, the kind of environments that throw you into unknown code often change your project so fast that any attempt to understand the system more deeply will be soon lost as you change project again. This might be a nihilist view, but lots of coding jobs are essentially exercises in futility anyway, so might as well be efficient about it...