I am using QField for field data acquisition. Can QField connect to an external Bluetooth GPS?
If you want to use an external GNSS-Reciever on QField in your (Android?) mobile you need to connect your mobile (phone or tablet) to the GNSS using a separate app that provides a mock location to your device.
The app you should use depend on the GNSS brand and configuration. The official QField guide suggests, as for example between several options, Android NTRIP Client.
To use this you have to enable mock locations on your Android device: e.g. if you are using a Leica a good app is Leica Zeno Connect, while Stonex works with Cube Connector.