At my company code is required to be self-documenting. My attitude is that if you can't determine the why then you likely are not familiar enough with the problem domain to be working with that code. It's fine not to be familiar with the domain and there are ways to address that, but reading source code is not one of them.
Yes absolutely. All developers, junior and senior, go through a 4 month training program working on a completely independent project from scratch that teaches them everything they need to work in their domain. There are exceptions now and then, but for the most part it's pretty consistent.
When a developer wants to switch from one area to another, they go through an accelerated program (takes only about a month).
