I'm no expert but it's my understanding that on machine guns like the MG42, the bolt pushes the cartridge out of the belt and into the chamber. The action of the bolt assembly moving back and forth advances the belt. To get an MG42 ready to fire, you pull the bolt back to lock it, open the top cover and lay the belt in. Here is a video of the M60
which uses a close copy of the MG42's feeding system.
On older machine guns like the Maxim, I think the cartridge is pulled out of the back of the belt, dropped down and then pushed into the chamber. You have an extra action to get that style of MG ready to go but it's been a long time since I shot either style.
edit to add: About nine and one half minutes into the video they have a big clear plastic model they use to show how the m60 works.