Hi there,
It seems all meshes are not parented to a main mesh, similarly to the car rigging, where you have Chassis as the top-level mesh, and everything else is parented to it. Objects that will move with a left door should be parented to it. The same logic applies to the bones in your rig. However, if you are using a rig, you don’t need to break the mesh into several parts, because you can have a single mesh split into several vertex groups (edit mode).
Also, if you are not making animations in Blender, you don’t need skeleton rig in Blender for rigid bodies. At least, not for a vehicle. All you need is to set the meshes with proper pivot points and parented one to another forming a mesh hierarchy. Unreal Engine will rig automatically for you if you import as Skeletal Mesh.
To parent an object A to an object B, select the object A, shift select the B, CTRL+P and select parent to “Object”.
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