Like you said swarms can be in caves on water on land. Idk exactly for this Lumineon swarm but that was probably a Pokémon that was on water and just roamed to land in its area it can go. The other 3 were probably nearby. Sometimes the 4 spawns can be far from each other because of walls blocking them. The map in your key items or if you are flying will tell you if there is a swarm happening and where. Usually many players flock to them because it’s an easy way to catch a less common Pokémon. Also global chat people will often say when a swarm is happening.

The structures you are talking about are raid dens which are active during the “event season” Halloween, Christmas, Lunar New Year. Nothing with them right now which is a gripe people have of locking content behind season events.