QField is an open-source Android based mobile data collection and/or viewing application that is tightly integrated with QGIS.
QGIS is used to set up the project using standard QGIS tools.
If the layers in your project are stored in a PostGIS database and a mobile data connection is available then changes made in QField are made directly to your database and visible in real-time to anyone that has a client to your database. This, in my view, is the mobile data collection killer app.