I forgot to mention. I use Vue, been using Vue since version 5. Bad news is it is being sunsetted. Good news is it has been made totally free even the previously very expensive maya/c4d integrations. Makes great Skydomes, HD light setups, the works. Fairly straightforward to put what you make in to Unity from there.
Unlock the legacy of e-on software! Enjoy free downloads of VUE, PlantFactory, and PlantCatalog.
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You can always start with SketchUp.
If you want to mash together free assets you find the best solution always has been and still is the free version of meshmixer. The paid corpo version of it is called Fusion but it really is not necessary.
All are free but are still arguably the best at what they do and are fairly simple if you've used any 3d program before. Sketchup doesn't get much simpler.