Can anyone help me understand the difference in shading on these 2 models and how I can get the rough one smooth like the smooth one? It's not just shading flat or smooth.
The left bush (the smooth one) has Custom Split Normal Data. You can delete it in Object Data Properties > Geometry Data. There is a button to remove it. Then the bush looks like the bush on the right side.
If you want to have the right bush look like the left one then add a Normal Edit modifier. Use type
Radial (default) with Mix Mode set to
Copy (default) and a Mix > Mix Factor of
0.950. In Object Data Properties > Normals the Auto Smooth checkbox must be enabled.