I believe to see other addon aircraft, both players must have the exact mod and liveries installed. Those
mods must also include the multiplayer version of the aircraft/liveries. So if someone else is flying the same developer Typhoon for example, but with a livery they don’t have, it will show a generic.
Also in Options>General Options>Traffic> Use Generic Aircraft Models(multiplayer) must be OFF, and Traffic Variety must be Ultra (not 100% sure on this part, but probably so, as it would restrict the available liveries if set below this)