Most games (pre-ai at least) would use a brush for this and manually tweak the result if it ended up weird.

E.g. if you were building a desert landscape you might use a rock brush to randomly sprinkle the boulder assets around the area. Then the bush brush to sprinkle some dry bushes.

Very rare for someone to spend the time to individually place something like a rock or a tree, unless it is designed to be used in gameplay or a cutscene (e.g. a climable tree to get into a building through a window).