I've worked at a variety of places and have never seen any kind of system or architecture diagram going in (should probably ask about this in the interview stage). Further, it often seems that no one understands the overall system well, in some cases even when there's an architect.

So I usually find myself in the same position of using jump-to-definition and trying to get a handle on things that way.

From there, I'll sketch a simple diagram on paper to aid my own understanding of whatever piece of the system on working on. Personally, I'd avoid software diagramming tools at this stage. A simple hand drawn diagram can be extremely helpful and doesn't take long to create.

After a while, I'll often start sketching out a more comprehensive system diagram with the database(s), processes, etc. At this point, I'd ask management about working on this because it can be a time consuming process that involves talking to a lot of people.

In my experience, this has usually been an uphill battle and somewhere between difficult and impossible to take on as an individual developer, but I think it's worth a try. Taking this kind of initiative could lead to career advancement, but I've often found that there's a lot of resistance and it's not seen as a priority.

It might seem a bit cynical, but as a junior or even intermediate developer, I'd also advise caution against stepping on anyone's toes. I'm not sure why, but some people tend to get defensive about this kind of thing.