How to get started on programming devices from scratch

Hello,

I have not been programming for long, and in C for even less time, but I really really love doing it. I've programmed for a few different devices, and wondered if I could do something in robotics, but I'm still in high school, so I do not have enough money for having that as a hobby.

But it made me wonder; there are thousands of cheap devices on the Chinese import market like AliExpress, some are as cheap as a buck and give you a device that, although not very powerful or properly finished, should be able to run a few programs.

I know people have programmed for iPods (Rockbox for example), so the same *must* be possible for Chinese imported MP3 players. I would love to import something for a buck, and use the power of programming to make it into a Pong player (I make Pong games. They're my Hello World program.)

Are there any resources on how I could get started at this? I believe it would be the same as flashing Android devices, but different. Or would those Chinese devices run a custom kernel that would require a lot of work to access? I can't imagine a company making a 1 dollar MP3 player would care about security. Although they probably don't care about user accessibility either...

I know I'm not an experienced programmer, but don't hold back books for more advanced people, I can throw out 1 dollar to try something, even if I fail I would learn something.

Thanks in advance!

By the way, wasn't sure where to post this, and since I sincerely hope it's possible to program in C, I'm asking it here.