"My teacher told me that there are quite many companies design their products by C. Is that true?"

Yes. I suspect that very few companies would consider starting a significant new project in assembler.

If you are trying to decide whether there is any point in learning 'C' the answer is yes. Once you become proficient in 'C' you will be able to complete most embedded projects more quickly than using assembler. In addition you will be able to write code for any microcontroller (assuming a 'C' compiler is available) without having to thoroughly learn its instruction set.

There may be occasions when you will have to resort to assembler for a small (usually time critical) part of a project but these should be rare. In fact, the only occasions where I've had to use assembler have been in PC projects many years ago.

When you first start using 'C' you will write a lot of code that doesn't work properly. When this happens take the time to find out and fully understand why the code doesn't work. Don't just change it around until it appears to work. If you follow that single piece of advice you will eventually become a good 'C' programmer.