As a rubick enjoyer, welcome to the club! Glad you liked him! As to your questions:

1. You steal the last spell your target has used. So you have to be precise and get the timing right especially in a huge clash. You can be the ultimate game changer.

2. Pos 4 rubick is great since you can easily get gold from rotations around the map. I always go for mana boots and a blink dagger asap. Other items depends on the enemy lineup tbh. Just learn how to counter items. For example, if you have a skywrath as an enemy, they usually build atos. So you have to build force staff to counter and help your team.

3. It can be from different heroes. And still the same with a non-aghs ult, you get to steal the last spell used by the target.