Hmm, how do you know you understand programming?

I have programmed for 30 years, more than 10 in the area of embedded Linux, mostly user space, but also kernel and boot loader. My code has been running in at least a million devices in the field. However more often than not when I need to look into systemd code, I have the feeling I have no clue how this stuff can possibly work, have I ever learned to program?

I would argue that this ‘humble; always learning’ mindset is exactly what enables experts to become experts but ironically causes experts to feel like they’re not experts.

Yes you did, and of course you have a deep understanding. However, nobody knows everything about something, even if he studies that something for a lifetime.