How to pass variables a and b received by JS from website to Arduino?

Question

Arduino

The site sends a certain number, let's say 15, it must send arudino the value 1 and 5, that is, the numbers are separate, how best to implement this through a POST request, the arduino is connected to the computer via USB

Answer 1

The site is a server. He doesn't send. It only responds to customer requests.
The Arduino client sends a request to the site, and receives a response from the site.

Answer 2

And in general, is it really possible, if somehow by creating a web server based on arduino, or can you somehow do without it?

Answer 3

...arduino connected to computer via USB

therefore, the computer makes a request to the site, the computer will also receive the answer (more precisely, the program on the computer). Most likely, you will have to write the program yourself, so the easiest way is to entrust the analysis of the answer to this program on the computer side.