OpenSky works out of the box as a flight data tracking source and is enabled by default in the Settings > Input Channels:
Enables usage of OpenSky Network as a live flight data channel. As this channel provides positional information only, OpenSky Master Data (see next row) is forced to be enabled, too, as long as OpenSky Network is enabled. If interested find API documentation here.
This link opens the OpenSky Explorer, in which you can check current coverage.
OpenSky Network Master Data
For fetching master data on the involved aircraft (aircraft type, registration, operator, ...) OpenSky requires this additional channel, but it is also queried for flights read from other channels. So generally, it is a good idea to keep it enabled.