iPhone satellite SOS communicates to a constellation of 24 GlobalStar Gen 2 Low Earth Orbit satellites, orbiting somewhere between 800 and 1000mi[0]. This is the same satellite system that SPOT messengers talk to (which are also tiny devices)[1].
The user points the phone at the satellite (in reality, the UI tells the user where to put the phone in relation to it's measured antenna pattern to maximize the gain towards the nearest satellite), while the satellite has a huge, very very high gain antenna array to pick up the signal and pass it back down to a ground station. iPhones can output up to 2 watts of RF power, which is enough for a tiny <HELP! Here's my LAT/LON & status> message. It's using 5G NR band 53 [2][3]
I'm sure someone out there has already run a linkbudget and posted it to their blog, but I haven't found it yet.
1 whole watt is a lot by modern, digital standards, but satellite does not have a directional antenna with gain to receive your signal.
It looks like Garmin InReach transmits at 1.6Watts. I wonder what the radio is inside the new iPhones.
When you're going straight up you don't have to deal with obstructions and the curvature of the earth.
For OP if you want to see it in action there are plenty of YouTube videos[0] of amateur radio operators with an HT (walkie-talkie) contacting astronauts on the ISS - which is 254 miles up.
