How do Pillager Patrols Spawn?

You can find all the conditions in the official Wiki. For Bedrock edition:

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.

Patrols spawn around 24-48 blocks away from the player (or more than 48 blocks away if the player is in a village) in any biomes except mushroom fields, mushroom field shore, nether and end.

Additionally, patrols can spawn in biomes where common monsters cannot spawn such as in: river, frozen river, snowy tundra, snowy mountains, legacy frozen ocean.

Patrols spawn on solid blocks at light level 0–7 at the surface, like monsters.

If you never see any, maybe it's the mushroom biome, or your area is too well lit even during the night. Or, if you play with a lot of other people, they may be choosing to spawn near them instead.

I play in a single player world, and I see those guys all the time. I live near a dark forest, so they always spawn under the trees. Dangerous place to wander off!