For the technical requirements to install and use the app, see here.
You can find all the SmarTracks Locations here.
No, you can use the app without a DX sensor.
To ensure good Timing Gate detection and step recognition, you need to wear your smartphone on your lower back while running. To see whether your smartphone is suitable for the app, see the technical requirements.
If you want access to livestream results and other features, you need a DX5.0 sensor.