It's incredibly easy to do (caveat - at least if you're familiar with software dev already).

Most thermostats are literally just digital thermometers that control a relay that turns the furnace/ac on and off.

A simple arduino (or much cheaper IC) can easily do the same thing if you wire it in.

And then on the software side... there's several large, open-source projects that exist in this space and provide nice api tooling for interacting with those devices. Things like:

OpenHab: https://www.openhab.org/

HomeAssistant: https://www.home-assistant.io/

HomeBridge: https://homebridge.io/

etc...

Even Alexa has basically drop-in self hosted alternatives like Mycroft: https://mycroft.ai/ or ADA/Almomd (now Genie) https://genie.stanford.edu/

It's not only true - I strongly suspect you can do it for much less than 50 bucks if you don't need the physical thermostat to have buttons/screens.

Makes sense. My setup doesn't allow for that. Hence my ignorance. Good for you!

I'm considering making an OpenTherm controller for my heating boiler, I just researched this topic a few days ago - it's absolutely true, there are ready-made Arduino libraries for that.