DIY Sonoff TH Smart Thermostat, How Did He Do It?

Seen this guy who seem to throw out his TStat and instead configure a Sonoff TH10 to act a tstat. I don’t quite follow the all important part, the nitty gritty of how he achieved that. Any one can expound, extrapolate…for us mere mortals? I don’t know electronics but I can follow instructions to achieve same result.
Vid Link To → DIY smart thermostat. Part 1 - Relay isolation on Sonoff TH

That is a pretty step by step video (especially when combined with part 2). If you don’t have a basic understanding of why he did what he did to the Sonoff TH or how a thermostat works, I wouldn’t be messing with the equipment, your likely to damage the equipment or start a fire.

I can appreciate that but already said I am not familiar with electronics but can follow directions very well. I have moded a bunch of sonoff basics but not the TH10. My confusion was the tstat part but I notice his is controlling a heating furnace only in a shop while I am looking into how to fit this concept into my house’s dumb tstat and control both heating & cooling whether from this one unit or whatever combination would make that possible. I came across 2 TH16/10 units while cleaning out my garage, and thus, my interest.
Here is my tstat

In the video you linked that person is simply closing/opening a circut (the relay on the sonoff) based on temperature. The attached heater is controlling the rest (starting the heat and fan as needed).

From the wiring shown, you are wanting to control your blower fan, A/C compressor and furnace heat. Some of which need to be turned on at the same time and some of which should never be turned on at the same time. A commercial thermostat controls these functions properly and is the correct solution here.

Of course but have an infrequently used extension unit that I wouldn’t mind to experiment with if this is doable since I found a few of these devices in garage. This has caused my tinkering mind & fingers restless curiosity.
I apologize but indulge me for a minute - theoretically or realistically how would this be achieved anyway to control blower fan, A/C compressor and furnace heat in winter, using said two sonoff th10? I suppose thermostat would be rendered untenable and th10 temp readers would sub for temp data input to sonoff?