Question about flight tracking apps

I use various flight tracking apps to assist my plane spotting. I was curious of a couple of things.

One, what is your favorite app for plane spotting?

Two, why do some apps show flights that others don’t? Do they have more clearance or more access to different databases? For example, sometimes I’ll see a flight on FlightAware that will show the airline and the flight number, and then I will look on Flightradar24 and it’s not there. I can find the plane if I know where to look, but it will say N/A for the flight information. This typically seems to happen with things like military planes, cargo planes, and helicopters, but sometimes with other airlines as well.