Make sure all your meshes have their bottom at 0… uhm, hard to explain.
When you make your mesh in a 3D program, you place it in 3D space. So you have to keep the bottom of your art work at 0 on the Z axis. If you have the bottom of your art in say -100, it is 100 units below 0.
Take a look at the “material sphere” mesh in the standard assets. it is placed with the bottom below 0.
Then take a look at the bush or chair. They have their bottom at 0.
as for putting them on the ground.
do 2 line traces. one from your camera 250units out, (what you’re probably doing right now), and then a line trace straight down(just maintain X and Y from first line, -1000000 on Z). Where this trace hits you place it. then you have a point a set distance away, and straight down to the ground.
Easy peasy.