It's a paid-up-front app, so you buy it through the App Store, which means you can use it with any device you have that's signed into the same App Store account. I don't see any in-app purchases.
The devices don't have any GPS. Instead, they have a unique ID that phones see, and report their location to Apple/Google/Tile. If the network is bigger/denser, then your devices is more likely to be detected, and detected more often.
Firefox removed built-in support for RSS feeds years ago. When I try to view the feed in that image, Firefox just downloads the RSS file (sometimes Firefox will show me the raw XML instead). You probably have an extension installed that styles them.
I'm not sure why you're being downvoted when that's exactly what's happening. The QR code comes from the mask-image in the inline-style, not the video.