The Blaze are Jonathan and Guillaume Alric, two cousins  both about 30  years old, they say  who live in Paris. Born in Ivory Coast, Jonathan returned to France as a three-year-old, where he and Guillaume both grew up in small villages, surrounded by the classical music their parents loved.

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.