Connect RPI with Bangle and send data

are (some, data) variables passed to the function?

Yes, you can pass literally anything, you just have to turn it into a string. I'd suggest using Python's built-in functions for creating JSON, so then you can just define whatever Python variable you want, then send the command "X("+json.dumps(my_data)+")\n".

Does myapp.boot.js run in the background

Yes, any '.boot.js' file will get automatically loaded along with an app, so you'll be fine without an info file. And if you want to add your code to the app loader all you need is something like this:

https://github.com/espruino/BangleApps/blob/1d7f2070cffd318b74a42c0392addf48f3dca5fe/apps.json#L2250-L2262