I'm self taught when it comes to computers and software development. For years before I landed a paying development job I did a lot of hobby projects. When I decided to take the leap and landed my first development job I took a fairly steep cut in pay. I was single, could afford the cut and was doing something I really wanted to do. It got the experience I needed and after the first year and changing jobs, my pay substantially increased.
I realize not everyone can take the approach I took. It may not even work these days. I did this 38 years ago when the industry was a bit more accepting of developers without a college degree.
Addendum: I also networked. I went to the equivalent of meet ups of the day. Talked with other developers, showed them my work, etc. This is how I found my first job.
With ~17 years of experience already, start with the study of the structure of C programs. Recreate some of it manually, build it, and research the things that do not behave as expected.
Bonus of using an open source kernel is they have a lot of eyeballs on them. They will be pretty dialed in versus studying random Github projects that happen to be written in C.
Would recommend avoiding cognitive overload, wait until you get into comfortable flow writing, building, fixing as needed, simple programs before you dive into lower level debugging, trying to grasp assembly structures that a compiler spits out.
LLMs can also help you break into C development by a large degree. But they still get overwhelmed on a sufficiently large C codebase just like any other language. Your mileage may vary there.
K&R is a great intro.
If you want a book that digs deeper, try Modern C by Jens Gustedt. It’s update for C23 this year, and it’s CC (free).
If you want to make something like TigerBeetle or Redis, that’s sort of just specialised back-end development.
You can also do embedded in C. And you can make operating systems.
What people often do when they want a career change is: they work on New Thing in their spare time; they find this little niche at work where New Thing could fit; they find that there’s a limit to how much workplace will let them prioritise New Thing; they start applying for jobs where New Thing is a focus point, with a non-empty resume.