@aceindy Thanks a lot!
It seems I managed to install everything and compile a firmware (I followed the wizard screens, not sure if the right platform was taken by default by doing so, as I’ve been asked to select only the platform (ESP8266) and the board (ESP01_1m), together with the wifi info and OTA password)…
The firmware has been generated and I also flashed it using the modified ESP8266 (I had to install the driver first though, otherwise no COM port was showing up (downloaded from here: https://parzibyte.me/blog/2020/02/09/instalar-driver-esp8266-windows/).
The flash went also fine, it seems 
Now, before connecting the little thing to the AC split, is there a way to check if it’s working as expected?
I have the other ESP8266, not modified, and I should be able to read from it. How could I do it?
Again, thanks a lot for all your help!
UPDATE:
I just checked the connected devices to the router and I can see that the little thing is indeed connected
Next (last?) step I suppose is to connect the ESP to the AC, I’ve got the voltage converter and some cables to connect directly to the 4pin plug of the AC. Let’s see
UPDATE2: I checked the AC connection and I have one question and one problem:
- The question is how the wiring should work? I have only a 5v print on the board of the AC where the USB cable was plugged, nothing for the rest. How should I connect the wires?
- the esp8266 and esp01 together are too bulky to fit the little slot where the usb adapter was. I will have to cut the black support housing the 8266 pins, let’s see what happens…