How does the NHS Covid app work? Why you get told to self-isolate

The UK government is considering decreasing the sensitivity of the NHS Covid-19 contact tracing app due to rising concerns that people are deleting the app amidst a surge in case numbers. The app alerts users to self-isolate if it registers a close recent proximity to a Covid-19-positive individual. Critics warn that the risk of being told to self-isolate is discouraging people from using the app, potentially hindering the ability to keep track of virus spread.