On the main screen, at the top is a black square. That means telemetry is not being recorded. If it changes to a flashing circle, telemetry is being recorded. Something must trigger recording to start. If you have not set a switch to start it, the default is "auto" which means that it starts when your timer is started.
Files are named and therefore sorted by, the date and time of occurring so it is important that the transmitter date is correct and that transmitter time is reasonably correct otherwise you will be looking for a log file that is named with the wrong date.
What parameters are recorded is up to you, by default all are recorded but you can switch off recording of ones that you do not need.
There is a max number of parameters that can be sent on telemetry and it is not difficult to exceed, which means some parameters are simply not sent. That is why the newest devices e.g. Assist Rx allow you to switch off unwanted parameters and thus reduce the telemetry overload