Hi @jakubdatel
From the questions you ask, it seems you might be quite new to this sort of thing. If you only want a board, I think you'll find that buying a Nano Every is the best way. The price is quite reasonable, especially if you buy the three pack.
If you are really interested in making a DIY Arduino board, I would recommend starting with an ATmega328P-based board. There are many tutorials about this. I can recommend this one to get started:
That is only one of the functions it serves. The other one is to act as a UPDI programmer used to flash the sketch binaries you upload to the board.
Instructions are available here:
See the schematics and board design files provided by Arduino: