I have an idea of a physical game supported by a mobile app. The key technical trick of the game is to check whether two phones are positioned close to each other with reasonable precision (let's say on a distance of 0 to 15 meters) or not (no, not another Tinder).

As I experienced, a typical GPS on a typical phone can easily by >20meters off at a certain time point, so two phones can be actually next to each other and appear >40 meters off, or vice versa, so it doesn't work.

So, the only option is to use Bluetooth. Here comes another challenge: would it be possible for two phones to check whether they are close using Bluetooth without being coupled and given special permission earlier. No other data transfer is needed, just the proximity issue.

It is possible. I can't explain how because I didn't do that on my own yet, but there are all those COVID apps which can detect people getting close together. There is at least one which is open source on GitHub. You may need to have a look on how they did that