And how would the is-my-partner-cheating get their app onto the victims device to detect the other apps?

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?

Which even halfway credible app developer would sell you that info? You know that’s illegal right? You might get some stupid indie developer to do this but no chance for anything even half big.

But if you can get actually get this data, maybe try to do this on yourself and write a blogpost about it. I highly doubt you’ll be able to.

I've never made an iOS app and don't have plans to. But my assumption is ~every >= medium-sized iOS app would be monetised by selling data to aggregators.

Even if that was the case - which it isn't - the aggregator data isn't keyed by the user in question. That is highly illegal pretty much everywhere and would get you in a lot of trouble. You can't "just" find out which apps an arbitrary person has installed on their phone. That's not how it works.

My understanding is it's common practice. E.g. How Shady Companies Guess Your Religion, Sexual Orientation, and Mental Health And sell that data to the highest bidder. https://slate.com/technology/2023/04/data-broker-inference-p...

Most app publishers are halfway credible at best, so it's not much of a problem. Even the halfway credible ones often use SDKs that do this.

Ok but if the SDKs do this they use it themselves to serve ads and don’t sell the raw data, right?

Get your hands on a random selection of 10 iPhones and look at the apps installed. I suspect you’d be horrified. As an example - any parent who has installed a free game for their kids likely has all of this info, plus more via tied in logins.

That said, I agree with the rest of your point - you’re not going to go to a developer and offer them $100 for this data on a person (and if you could, you’d still need to tell them which person, which if you could do you could just get the data yourself)

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.

If you can get the app onto my phone in person, you can also just check which apps I have on my phone

That assumes continued access, which may not be true. Installing spyware gives you information down the line.

But if you have credentials and physical access you can just ask for their phone and straight up read their messages/apps.

Yeah, once, possibly under time pressure, and not at all times. Spyware gives you continued access.