I've always felt that in the various jobs I've held the degree to which I could "get under the hood" was determined by people higher up the chain of command than me.

Run into a complex bug in library X? Well, we don't want you spending a week to debug it, write a patch and try to upstream it. Work around it for now instead. Or we'll defer that feature since we don't want you to spend that much time on "non-company" projects. Etc.

I think this is usually all the more true when your employer expects you to be a generalist of some kind and I've become a bit jealous of people whose employers let them really dig into things and even deviate to ecosystem projects instead of focusing on keeping the internal hamster wheel spinning.

On another tangent, one thing I found very frustrating early in my career and which I still feel is problematic in our field is that there's kind of a double standard when it comes to approaching things "under the hood."

At my first job I'd always ask my supervisor when when things lead there and then get told not to go there (despite wanting to) while a coworker of similar experience hired at around the same time would constantly get everything done behind schedule because he just did the things under the hood I made the mistake of asking about, but he'd get applauded for it continually. (Perhaps I learned from this, but it was pretty frustrating and disheartening at the time.)