The detail on the side of the ship and the stuff on the back, around the 6 round intake shafts, was kit bashed. Kit bashing means buying dozens of tank and car model kits, and gluing certain parts form certain kits onto the model. When ILM build the model of the Millennium Falcon model for The Empire Strikes Back, they used certain model kits. Of course, nobody remembers what kits they used. They just spent thousands of dollars on kits and took whatever parts they thought would look best. Since I was trying to make my Falcon as accurate as possible, I had to figure out what kits ILM used 20 years ago! But I was lucky. The companies (mainly Tamiya) they bought the kits from still exist and they still make MOST of the kits.
How do you find the right kits? Well, you go to a hobby store, get a catalog with pictures of tanks and cars, and get a book (from Star Wars to Indiana Jones or the Star Wars Chronicles) with pictures of the original Millennium Falcon model. Then you compare the pictures and see if anything looks familiar...! Yes, it DOES take a while! :-) But sometimes when you find a part from a, say, a tank you buy the model and find another 10 parts or so that you will need. (then again, sometimes you don't. So you can easily spend 30 bucks on a tank model kit and only use one part from it!)
The big ribbed part next to the cockpit for example is a transmission from a 1:8 scale Porsche which I was not able to find. Consequently, I had to build it from scratch. I ended up gluing 140 pieces of styrene together just to get that (the other half of that transmission can be found on the other side of the Falcon. It took another 220 parts to build that one...)