They don't, utilise the fact that every single iPhone app has access to what other apps are installed! - purchase that info from literally any iPhone app or aggregator that has it for that user. Curious how much this would cost to purhcase - a working credit card goes for $5-10 on the black market so 'apps installed on X's iphone' might be, like, 10c?

Ask any domestic abuser. Most of them seem to be successful at it.

https://www.npr.org/sections/alltechconsidered/2014/09/15/34...

It’s crazy to me that people are being so skeptical of the idea. A lot of people share their logins freely with their spouses. I have never done it nor would I condone it, but it would be trivial for me to install spyware on the devices of many people I know, because they rightfully trust me. Not only do I know some of their device passwords¹, being “the computer guy” I could just outright ask for it or get them to input it anywhere while fixing some issue they have.

¹ And many more I have forgotten, because I make it a point to not record them, even mentally.