Primarily my brain and upper body limbs. With a little help from Procreate on an iPad Pro. I do a little colour and brightness editing in Photoshop as well, but it's a pretty minor step.
I'm betting want more information than that, so I wrote a whole post called What App is That? for you.
Lots! Have a look at the Resources page for a collection of courses, learning platforms, and books I personally recommend.
You might also want to read my essay series on Drawing Invisible Programming Concepts
You can indeed print any images you find here for your own personal use!
The image quality on the garden notes and twitter is low for printing (72dpi). But if you join my email list I send out high-quality, downloadable versions on there (300dpi – big enough to print on the side of a wall if you so desire).
I'm not fan of the carbon emissions involved in printing, storing, and shipping these from a central location. So if you want professional prints I suggest finding a local printer near you and sending them the high-res files.
I think about this too much, but I have three main words I use: illustrated notes which do what they say on the tin. And illustrated explanations or illustrated essays for longform work I spend more time researching and crafting.
What I don't make...