EDIT: Pillagers continually spawn in a 72×54×72 block volume centered on the top floor. They may spawn on any valid opaque block as long as the sky light level is 11 or lower, and the block light level is 8 or lower, but can also spawn on grass or sand blocks regardless of block light level.

What causes Pillager patrols to spawn?

Patrols spawn naturally after the world age reaches 100 minutes (5 in-game days), then after a delay between 10–11 minutes an attempt is made to spawn a patrol with 20% chance of success. After an attempt is made, the delay is reset.

How do you get RAID pillagers to spawn?