It's a lot less magical when the tricks are revealed, but build your base scene, render it and then create the additional parts via utilising Spot Render (you can select "New Window" within the Spot Render tool so you can save out that individual render).
And then you take it all over to photoshop and align them accordingly, build up your scene and voila. It takes slightly more time, but less demand on your PC and you can render certain parts of a scene rather than constantly rendering the entire thing = speedier renders.
I do use billboards in some places, but I prefer actual shading/lighting bouncing around where billboards cant really do that.
Treat your creations/see your creations in layers, and you'll get things done much better
View attachment 1569837
