I've noticed that at Central Cavern the VTOL isn't always there. On Williamsburg the alien weapons are sometimes there, sometimes not.
I know that PC gamers can have customized server configs, but how is it to explain on the xbox?

Alien weapon selection and placement is randomised. The VTOL can be shot down and has a respawn time.

Allright thanks. But the alien weapon selection & placement aren't random as far as I've seen. When they appear it's always the same weapons at the same locations or nothing at all.