Shimano is the manufacturer of the components that went on the bike. Kind of like if Ford were to make a car and use gear and motor and wheels from a separate company and Chevy could also buy the motor and wheels and everything from the same 3rd company. The Shimano 600 would tell you the quality of the motor and wheels and everything (in this case brakes, gears, cranks, headset), but won't tell you the brand or model of car because most companies from that era bought from them (or 1 or 2 other companies, but that just complicates things). Just that 600 wasn't the cheapest or the most expensive level of parts so it wouldn't have been put on the cheapest or most expensive car/bike, more of a mid-level (so basically you're not looking at a Ford Pinto or a Ford Mustang or a Chevy Metro or a Corvette, somewhere in the middle). Schwinn takes the place of Ford or Chevy in this example (if it makes any sense at all) because they built the frame and put the parts on it, so we still have to figure out what model of bike it is. Likewise, the cinelli on the bars (another component maker) probably tells us that it isn't something bottom of the line. The headbadge would probably say Schwinn Paramount (unless they'd stopped doing that by this point) if it was top of the line, so again this is somewhere in between.

I will say that if you're looking to resell it, the fact that it's such a large size will probably work against you. I'm a tall guy and there aren't many bikes for sale that fit us, but at the same time there aren't many of us that ride bikes, so you might find one tall guy desperately trying to find a bike that will fit him, but it would take a while and tall bikes tend to sell for less than medium sized bikes or small bikes (that often go for a little more for the same make/model).