We can roughly distinguish between two types of COVID-19 apps: symptom checker apps and contact investigation or tracking apps.
A symptom checker app helps to determine if you are possibly infected with the corona virus. Users fill in health information via the app, for example by answering a questionnaire. A remote medical team or manned call center assesses the complaints. If necessary, a healthcare provider will contact you by telephone within 24 hours. Examples in the Netherlands are the Corona app from the OLVG and the Corona test from Thuisarts.nl.
Tracking apps can contribute to contact research: they help track corona infections. For example, this works as follows: when the app registers that app users (all of whom have Bluetooth enabled) have been within two meters of each other for at least fifteen minutes, a note is kept of this. If one of the users is diagnosed with COVID-19, the app requests permission to send a notification to other users of the app. Everyone who has been in contact or in the vicinity of the infected person will receive such a notification via the app on their own phone. These users are invited to take precautions and are tested themselves if necessary.
Contact tracing via an app is a proven strategy that scientists previously used to combat Ebola or HIV outbreaks, but does raise questions regarding data and privacy.