As I understand it, it logs the MAC address, and location, of anyone you have been in close contact with for (I think) 15 minutes or longer.
It holds that info for 21 days (incubation period plus a few days).
If you or someone you have been in contact with, is diagnosed with Covid, you will receive a message to present yourself for testing.
If there is no call from the command centre, your outdated data is deleted.
Don't know what everyone is worried about. Companies and governments have much more, and way more private, data on you without needing this app.

