Live Traffic is fed to the sim by Flight Aware, a commercial flight tracking service - flights that are happening in the real world right now, delayed by about five minutes. That includes jetliners and small aircraft.
Offline Traffic is a combination of commercial and small aircraft as dictated by a built in schedule inside the sim. It’s 100 percent sim generated. That’s governed by the Traffic Density slider setting.
Live Traffic will always be less in volume than even the lowest of Traffic Density settings using Offline. That’s because in the real world, tracking of flights end-to-end, from gate to gate, isn’t 100 percent reliable.
Departing Traffic is broken because the aircraft are missing nosewheels. There is an open bug report you can participate in.
There are workarounds, but notably they either involve cloning a copy of the departing traffic and giving if nosewheels (which has sometimes unexpected results) to forcing departures using AI Offline Traffic (which works quite well). I prefer the latter myself, using Aerosoft Simple Traffic. They add the right liveries (airline paint schemes).but use Asobo’s already provided generic plane models. Even at 30 percent density, I have tons of aircraft around me for the 80 NM range the sim generates around the player aircraft, landing and departing, without having to finagle with external programs or downloading large amounts of liveries and model files. YMMV as they say, but this keeps my sim installation small and simple to troubleshoot.