It is possible to create VRMs using different methods. UniVRM and 'VRM for Blender' are two commonly used tools to create the VRM itself.
'VRM for Blender' is an add-on for Blender that allows you to create a VRM without needing to install Unity at all.
One disadvantage to this however is that if you are planning on using Spring Bones, you will not be able to see the results of this without uploading it to a VRM platform. Materials are also a more difficult using VRM for Blender.
UniVRM can easily test the Spring Bones as well as add materials easily within Unity.
Rigging using the 'VRM for Blender' Skeleton
'VRM for Blender' offers a skeleton to use for your asset rather than making your own. You would need to manually rig, or paint the weights, of the mesh vertices to the skeleton bones yourself.
Rigging using an automatic tool such as Mixamo or Accurig.
Mixamo or Accurig allows you to save a lot of time on rigging your character, at least as a starting point. That said, you will need to make sure that 'VRM for Blender' understands the rig bone setup that Mixamo or Accurig uses before exporting. This can be done with the Bone Assignment tools in 'VRM for Blender' covered quickly in Exporting the Avatar.