Or does one just happen into these jobs as an apprentice and spend years learning on the job?
In the specific case of 802.11 (different from 'Wi-Fi' which is a marketing term from the Wi-Fi consortium), you'll also need to be familiar with the IEEE 802.11 specifications. You can do this by putting aside a block of time to read parts of the actual specs [1], supplemented by the O'Reilly 802.11 books by Matthew Gast [2] which I found to be really helpful during my time working with various 802.11 modules.
TL;DR is - I learnt on the job and as a hobby, and I came to it having some idea of kernels and networking already.
Long ago, Adrian got taken under the wing of a well-know FreeBSD developer.
As most other things it comes to passion. If you literally write C in your dreams and spend every waking hour hacking OS kernels you'll get to be very good.
do it enough times, and you too will write blog posts like these.