Ask HN: Can you directly program a smartphone?

I've been thinking about the following idea: Is there a way to directly program on smartphone hardware, at the same level of interaction you would have with embedded devices?

I'm talking in a pragmatic sense. Of course this is possible at a theoretical level, since at least the manufacturer does it. But can a random person run code on its device? Are there devices that make this easier than others?

This is more of a hypothetical question, so I'd like to find any reading material on the subject.