I'm fairly sure that's technical pen on paper which has been scanned, run through a quantize filter to clean it up, and shrunk.

For sure, although not completely impossible to replicate in software.

Balsamiq Mockups can make diagrams that look similar.

In this case almost certainly paper, technical pen and somewhat steady hand.

Drawing like this becomes natural on engineering paper or any generic graph paper with some practice.

Beats the heck out of any software for efficiency in communicating arbitrary ideas in particular because it is not an opinionated interface (personal opinions aside) and you can do what you imagine without limitation. When you have to scale to dozens of drawings or collaborators then the benefits of computerization show themselves, but not before.

With pen and paper the primary thing in my mind is the idea I'm trying to convey, with a computer it is how to convey it with the computer (until I've reached some level of expertise with the computer in a particular space).