How did they affix the gold to the boards?
How did they overlay the bolts (which had to pass through the rings on the boards)?
Rashi: Every plank had a ring affixed to it, and from the ring on one plank to the ring on the next plank they affixed a golden half-pipe (like a spliced cane), to cover the bolt.
What purpose did the bolts serve?
Rashi (in Shabbos, 98a): 'Because they (the boards) were tall and when they stood in the sockets, which were only one Amah tall, they would sway backwards and forwards, and it was the bolts that held them (each side) together; whereas the middle bolt joined all three sides together and held them firm.
How did the inside walls of the Mishkan compare with the outside?
Rashi: All the current additions to the boards were confined to the outside. The inside of the Mishkan was smooth.