@nameLess I must admit I am everything but expert about #OpenTherm. I've just started digging into it recently and I have only basic understanding. I have some impression, which I am going to share below, but please remember that I might be wrong. Please consider that as a good starting point for further research. If you find anything valuable I'd be glad to know. Especially if I am wrong.

So, we have following entities: gas boiler, thermostats (sensors+valves' actuators), central unit (ST-2801 or #TermetComfort), and #HomeAssistant. The central unit knows from programming what is the expected temperature in given room at the time, and from sensor what is current temperature there. According to the data it opens valve and turns on the boiler. The communication with the boiler goes either using on/off method (as you have called it: short circuit) or with OpenTherm protocol. The 2nd way allows to use the boiler more efficiently. You do not need to heat the water to 70°C if you just need to raise the temperature in one room by 0.5°C. Also, the central unit can read back some parameters from the boiler (required water temperature, water temperature on output, temperature of returning water, is DHW - domestic hot water - being provided at the moment and a few more, depending on the boiler).

The central unit - depending on the producer - may or may not allow to connect a temperature sensor by wire using OpenTherm protocol. However it is unknown to me, which device is superior. OT operates in a way, that one device initiates a communication and either ask the other device to perform something or polls for current state. In case of controller/central unit and gas boiler, the first one is controlling the communication with the other one. I have no idea how it works with OT-enabled sensor and central unit. So, there is a chance that you will be able to talk from HA to the central unit using OT integration, but I do not have enough knowledge to tell if that's possible.

As for HA integrations. Apart from officially supported ones, there is a service called #HACS (Home Assistant Community Store). It provides many add-ons, extensions and integrations. However, it seems the only way to find what is already supported there is to install HACS first, then browse it from HA level. So, there is a chance someone has written the Tech or #Termet integration and made it availabe through HACS.