Hello Guim 31
To quickly progress in node red, check out this video series, it explains quite a few things that helped me with the Xiaomi Gateway tutorial: [Course 2.2 Bioautomatics] Node-RED Interface - YouTube.
Then in the tutorial: [NODE-RED]Piloter la Gateway Xiaomi chinoise - #4 par Psoy
At the bottom of the tutorial, in the « Sensor Part » section, I discuss the extraction of data from a Json. You’ll find it in the « function node ». You’ll need to adapt your data.
That should do it. 
Your turn to play.