While Apple and Google work on a new cross-platform contact tracing API to help tackle the COVID-19 pandemic, we’re starting to see some countries release bespoke apps built using just the existing Bluetooth APIs on each platform.
What we’re quickly seeing in the media about these solutions is that the iOS app has some limitations while in the background (i.e. not the visible and active app), and as a result the creators of these apps are often recommending people leave the iOS app in the foreground, or that they work better if Android devices are nearby.
A great explanation on why contact tracing apps don’t work that well on iOS, and at a much higher level of detail than most publications have been reporting.
I would also say, that even if you’re not interested in the context around contract tracing, you can get a really good understanding on how Bluetooth works in general on iOS apps.