In re the snails in our tank:
Overstocking is not an issue: we had ZERO fish until two days ago (Friday 12 June 2020), only plants, and we added only three young/small gouramis and 10 0.5" shrimp to the tank (75 gallon).
Similarly, overfeeding is not an issue, as the fish and shrimp have been fed only one time (Saturday 13 June 2020) and only a tiny amount, as I was concerned about how much to put in. They finished off what I gave them in less than 2 minutes.
Inadequate maintenance is not (yet) an issue, as the tank is only about two months old. Plants were first added 13 May 2020. We allowed it to cycle until 12 June. Some cleaning, removal of plants that completely melted, and replanting was undertaken after about two weeks, around the end of May. Since then we've been monitoring the water chemistry and adjusting it.
It's simple: the snails came on the plants, and I missed seeing them. So far I've seen maybe a dozen. I have also seen evidence of eggs on the tank walls, so I know we're headed for a lot more snails if I'm not careful. Some are okay; hundreds sound overwhelming.