Blender
For those interested. Learning Blender is not always necessary. There are plenty of 3D assets, including free ones, that you can find on the Unity Asset Store or Booth.
As for making world geometry, using the free Unity add-on Probuilder is often easier and quicker, especially for prototyping, as you will see if you choose to read the modeling section for worlds in this compendium.\
Probuilder has its limitations, particularly in making more complex geometry and shapes (which you should not attempt at first).
If you want to make more complex geometry for a world, I would recommend making a simplified version in Probuilder first so you can enter VR sooner and get the scale right, then export that into Blender and make finer adjustments there.
^^ WATCH THESE FIRST ^^
Blender Basic Tutorials playlist <--- Good for learning specific tools, workspaces or processes.
Round trip world workflow: https://vrclibrary.com/wiki/books/maebbies-precise-solutions/page/blender-3-28x-mapping-config-for-mapping
What is a .blend1 file and why do you (not) need them?