but that didn't give clear explanation how main.c should look like.
I'd really appreciate any information where to find documentation about those components and guides for starting programming arm MCU's with keil uvision.
MCU: stm32F103C8T6
There is no standard. Do whatever you like.
Go into the pack installer inside of Keil, click BOARDS, and copy one of the blinky examples. Try starting with that. Esp with RTX, it's nice to get a main.c from the example so you can see how to start the OS and initialize the first threads. It's all very easy.
Now... If you want to move from the Std Peripheral Driver to HAL, there are a couple wierd things with Keil right now. Trying to combine Cube and RTE. It's definitely possible, but there are just a few gotchas with the order and whatever. Basically Keil and Cube are both trying to control things, and you need to chose one or the other. But if you aren't interested in the HAL, don't worry about that.