I’d love to know what the smallest OS is that can boot on an old MacBook, get to network (wired or wireless), and includes a basic browser (tui or gui) even if it doesn’t drive all the onboard hardware. That feels like it would be an excellent learning platform.

Not vouching for it working on a MacBook, and it sure ain't "modern", but you might love poking around in QNX. 1.44 MB was plenty for everything.

https://crackberry.com/heres-how-qnx-looked-1999-running-144...

These days, most people would run it in qemu, e.g. https://www.youtube.com/watch?v=xqILeQ-Cg-A

thanks - in that vein there's also freeDOS. but I assume if someone had built a learning OS they could make it much smaller?

haven't tried:

http://randomcomputerbits.blogspot.com/2008/02/tutorial-inst...

DOS just isn't much of an operating system, it's more like a bunch of helper functions preloaded into RAM, and you're free to do whatever you want with or without them.