they spawn within a certain radius of the spawner, which is within the bounding box of a nether fortress. But no, there does not have to be nether brick below them or the spawner for them to spawn. it depends whether you're playing java or bedrock.

All of the other humans (barring Santa Claus) were either killed in a nuclear fallout or were reincarnated as monster machines. AJ and Gabby are the only non-supernatural humans left. Because of this, they will have to repopulate the world when they are adults.

AJ is Blaze's driver, and is shown to be an expert at driving. While Blaze can drive on his own, AJ usually assists in driving him, often helping him steer in the right direction or stop him by slamming on the brakes. AJ can also operate the major parts of the machines Blaze transforms into.