“The more modern mound systems are the ones that are very susceptible to weight on top of the drain field”
“Mounded systems” aren’t more “modern” they are generally expensive “engineered systems”. These have to be install at great expense on properties that have slow perking soil conditions. A LOT of extra “engineered” dirt and rocks.
Be it an engineered system or a standard system buried somewhere out side the house. Nothing is allowed or should be built on top of it. Parking lot, pool, heavy equipment etc. This will compact the soil and cause premature failure.
“some of the neighbors have built a huge addition that I suspect is at least partially on top of their drain field, and the people up the street have a huge above ground pool right on top of theirs”
Unless they built without a permit, or the zoning and building code is very lax in your area. There is no way anyone is allowed to build anything even partially on top of a leach field. When an addition is being added or any kind of construction is being done. The building department usually make the contractors rope off the leach field area so no heavy equipment is driven over and or park on it. This is pretty universal across the land. Can save the owner a ton of money down the road.
“If your system is the older, conventional system”
The “system” design hasn’t change much for decades and decades. Pretty simple in design and function. Kind of like the original mouse trap.