Yes, yellows on 50 and both blacks on the same post.
Yes, ground is through the case, Aluminum is an excellent conductor.
The fatter Yellow is the solenoid circuit from the switch to initiate starting by completing a circuit. The second yellow wire goes back to the fuse panel area to initiate a device.
The blacks are battery power to the starter and to the rear fuse panel as well.
Google how a starter works, all over the net.
