Those $15 ones are also straight up scams. They just estimate (lie) for the readings based on other sensors.

And I believe the accuracy is also not great on these cheap ones. The product in the OP's photo costs $200 where I live! And ISTR finding the sensor itself contributes a lot to this cost.

IIUC they also need fans. The one I have in my home has one that's actually integrated into the sensor unit.