Slimes spawn below Y=40 only in specific chunks, and which chunks those are is fixed by your seed. The map overlays them so you can pick a farm site — ideally a cluster of slime chunks near your base rather than the first one you stumble into.

• Slime chunks are a Java Edition rule based on the world seed; Bedrock uses a different placement, so a chunk that works in one will not necessarily work in the other.
• Slimes still need the light and space conditions to spawn — a correct chunk with a lit-up cave under it will stay empty.