Basic FPS hand setup with multiple animated weapons

Hi,

I'm having a bit of a trouble for a few days now trying to understand how the workflow is between animating arms/guns in Blender and exporting the right for Unity.
My goal is to have a rig for multiple weapon animations / differnt guns.
I watched a lot of tutorials on YouTube, but they always describe how one gun is animated with arms. This then gets exported to Unity with all the meshes for the arms and the (one) gun in it.
As far as I know this isn't optimal.
I think i need one arm rig and just place the correct weapon in them and play the right animations for it.
Which brings me to my troubles...
I dont't understand how I can animate the rig in Blender and transport this to Unity so that I can build up my FPS hierarchy.
Do I export the meshes for the guns in the same file with the animations?
Do I put the gun bones in the same armature as the arms or do they have a different armature?

Right now my setup looks like this:

And my hierarchy like this:

I would now add more bones under the "gun" bone for different weapons and animate them.
But how do I continue from this when I dont't export all the meshes within the export to unity?