"So, do these guns use giant cartridges like field artillery, or loose powder, powder in bags or what?"
Well, multi part answer.
First, there are three ways of doing it, and a lot of it depends on the bore size of the gun.
First, there is the fixed cartridge. Most land and naval guns used fixed cartridges (projectile affixed to the case that contains the powder charge) up to between 4 and 5". That's varied over time. For example, the American 5"/25 gun, developed near the end of WW I and entering service in the 1920s, used fixed ammunition, while the 5"/38 gun didn't.
Speaking of the 5"/25, here's an incredible photo of the mounts on the USS New Mexico. You can see the fixed rounds, nose down, in what I believe is the fuse setting mechanism.
The British 4.5" quick fire dual purpose gun was about the largest gun of WW II that used fixed ammunition. It was chosen in part because it was about the largest fixed case round that would provide adequate performance and which could be easily handled by crew while providing a high rate of fire. Here's a British tar holding one...
Next comes the semi-fixed in which the projectile is separate and the propellant is contained in a brass case. The US 5"/38 that was so heavily used during WW II as the primary armament on destroyers and AA cruisers and secondary armament on cruisers, modern (and some refitted older) battleships and carriers used semi-fixed ammo. Rate of fire was about as high as the British 4.5" but could be sustained longer because the crews didn't tire as quickly having to manhandle the whole shell.
Next are the component guns in which the projectile is separate, there isn't a case, and the propellant is in bags. With 1 exception, all navies and armies during WW II used this type of load with guns of about 6" or more.
I'm pretty sure that all US Navy cruisers mounting 6" (light cruiser) and 8" (heavy cruiser) guns used this type of round as did all battleships. The US Army's heavy artillery, 155mm and larger, also used component rounds.
The one exception to the heavy gun rule of thumb was Germany. They never adopted the interrupted screw (DeBange) breech, sticking with the sliding wedge breech system that Krupp developed in the 1870s.
Because of that, all German artillery, even the heaviest (the 15" guns on the Bismarck and Tirpitz as well as their super huge land artillery such as the 42 cm Gamma Morser siege howitzer, used a separate shell with the propellant contained in a brass case. The case was required to seal the breech.
In some cases, such as the 38 cm Seigfried K rail gun, there were additional propellant bags loaded between the projectile and the brass case.
If you go here:
http://www.kaisersbunker.com/cc/You can see examples of various cases used by German guns during the WW I era with typical era headgear next to them for size comparison. Be sure to click on some of the larger ones. They're impressive.