This can be as “simple” as your phones being in close proximity to each other for an extended period, and sharing device advertising IDs/other device data via WiFi, Bluetooth. Might be more to it, but it’s a likely factor.

Devices do this regularly btw, smartphones also scrape for WiFi networks to better geolocate etc.

Yes but it also reopens everything exactly as you left it, meaning you can update and not loose anything mission critical; ymmv ofc but in my personal experience MacOS has the best update experience from mainstream OS