> good link on how the thermostat gets paired with the boiler?

You should have a book with the boiler that says how your system is setup. They nearly always include schematics and are very helpful. Typically you can open a cover and see the wiring details as well.

Forget about web sites, there are too many different ways a system can be setup, so even if they are not slop they can still be inapplicable for you. Once you know what you are looking at you can sometimes get useful information from the web, but until then you can't sort out what is useful for you.