If you're trying to track user information (notifications, actual timezone/language, battery level, VPN usage, etc) you can use screenshots of the current screen and open keyboard. You can also see stuff from other apps if the user is using split screen modes or has chat bubbles open. Apps can otherwise only access the data they render.

The research talks about thousands of apps but I do wonder how many of these are apps people use every day and how many are Chinese clones of freemium games and other shitware with a fraction of daily users. All we know from public app store data is the number of "downloads" and even that is distributed as a range. I doubt these 19000 apps were found by doing a survey on what people actually had on their phones.