1 - First question: is it right to put "ground" from booster to collector and base TO the ground rail of breadboard?
I did draw a simple (not correct, I know!) schematic just to render the idea.
It renders nothing, it is total crap. It is not even a schematic.Grounds have to be common for things to work.Have a read of thishigh-side-switching
Wouldn't it all be easier if you just used a sensor that can go lower? For example the the Si7021 or a BME280 which are (I think) more accurate then a DHT22 as well.
You mean with transistor?It has some pitfalls like phantom powering things etc.
It was for render the idea.
Sensor take voltage totally disconneted from the atmega
Half answering my questions, then I'll do the same to make it easy https://www.youtube.com/watch?v=2yFh7Vv0Paw