@Nina
Please allow me to utter a word of caution here. If you do not know what communicates with what and how, it is … , let’s call it ‘brave’ to start from a random point of view. As RussianDima said, the obvious choices are crossed out already. But it leaves a whole avalanche of other possibilities open: the communication could be done thru http-functions, a script running inside an html-window, maybe even the POS writes to a Google Sheet and your Wix site retrieves it using a webhook thru Zapier, just to name a few options.
In normal circumstances you would inquire with the original developer, but, I am taking a wild guess here, this venue is closed, otherwise you wouldn’t be here, on this Forum.
The sane course of action would be to have somebody with a background in Wix, to have a look at the site and code and give you a breakdown of what does what and where to implement the solution. Without this, you will be going around in circles.
If you need some reliable people to have a look first (this is usually not free), I can recommend some Masters (no, I am not available).