In terms of hardware, of course, datasheet is the best choice, and English is very important.

In terms of software, the embedded system has high requirements for C. As for the operating system, it should just call its interface. There are many ready-made things that can be used, and the source code will be consulted frequently, but it just stays outside the function interface (some protocols are complicated, but the system already contains them, such as tcp/ip). If you are interested, you can have a deep look, and how much you can accept is another matter.

I suggest you teach yourself, it will cost a lot of money, and you won't learn much after you go. The purpose is to find a job, so you should set up a device driver and expand it after finding a job. Linux device drivers are essential, and the author of this book has rich development experience and is worth reading.