Custom Challenges & Vehicles

I’ve been hearing about “custom challenges”.
Could someone explain how they work?

Also, two vehicles are missing.
What vehicle will act as the Warthog’s equal?
I know it’s one of these: Revenant, Spectre, or Prowler.
And which aircraft is in, the Falcon or the Hornet?

I dont think they are doing “equals” anymore. Not sure about the custom challanges but i doubt they are incredibly different from before.