How does a low power device make a direct uplink to iridium satellite?
It is a matter of link budget.

The people who designed Iridium did their homework and saw that the tiny fraction of those 2 watts that reached the big parabolic antenna of the satellite was sufficient. And for most cellular systems, you make cheap and low power terminals, while the base station (or satellite) has very expensive receiver equipment and high transmit power to compensate for the cheap terminals.

I found a very detailed paper on the subject, first describing the theory, then benchmarking the theory with actual numbers from Iridium. Rocket science level of the text though.