Weapon rigging like FPP gun

Hello there

so I’m new to modelling and I’m currently trying to understand how things work. I have a tiny bit experience with blender and I was wondering how Epic Games did the rigging for the FPP gun. So I exported it from the content browser and opened it in Blender, but I’m still not sure how they rigged it, because the mesh isn’t split up, but the bones are separately connected to the parts of the weapon (i.e. magazine). I played around with the armature a little bit and was able to place the bones to one of my own test weapons just like they did it with the FPP gun. I also added a root bone that contains all the other ones for the magazine, trigger etc. But I don’t quite get how they connected the bones to the mesh. The only way I know is to select the mesh, then the bones and press CTRL+P to set a parent. But my mesh is split up in several parts! And if I just join them all together I won’t be able to connect the armature to the mesh like they did…
When I tried to export my mesh and loaded it into the engine it also got imported as single parts which isn’t what I want…

Thanks a lot for your help, I hope someone knows how they did the rigging…Or maybe there is an even better way to rig it?