Why when I copy paste a variant, it becomes a component, rather than an instance?
Yes, you have to drag it from the assets panel if you want an instance of that component
Probably because this way you can duplicate the variant as a new component and quickly tweak each variant. If you want an instance, dragging from the asset panel is the way as previously said
Option/Alt dragging a variant to a location outside the component also creates an instance of said component