GTA mods give players unlimited creative powers, especially GTA Online mod users. Some mods incorporate people such as Iron Man while others allow modders to turn players into vehicles.
MattPageMagic and their friend are in a GTA Online server with modders which could mean a lot of things. It could mean there are new weapons, missions, and new vehicles. However, for MattpageMagic's instead of getting a new weapon, they are turned into a pickup truck with the legs of a human underneath. The modders, however, treated MattPageMagic's friend to an even better upgrade.
Flying through the sky is the newly upgraded Fortnite Battle Bus. The Redditor then jumps off the building that they are on to join their friend as they fly through the sky. Definitely not a normal sight in Los Santos.