How do I: Change which object is spawned based on the tile at the spawn position?

I have a Tilemap that looks like this:

I'm looking to spawn 10-20 objects every 20-30 seconds, depending on the specific tile at the current world position. I'm open to using some JavaScript for this.

No need for Javascript. You can check tile ID using "Tilemap Compare tile at" condition. Say, if it's a ground tile - spawn an earth creature. If it's a water tile - spawn water creature.

To convert between layout and tilemap coordinates use these expressions:

construct.net/en/make-games/manuals/construct-3/plugin-reference/tilemap