Lets start with the 1919...
If you look at the topcover, you'll see a slightly angled arm. At the top of the angled arm, there is a small circular post that protrudes out about .25" Now look at the bolt and notice the trach thats cut in it running top right to bottom left. The post on the arm in the topcover rides in that track. When the gun is fired and the bolt moves back and forth, it causes that arm to move back and forth (left to right) as it rides in the track. That arm, in turn, moves the feed pawl back and forth (left to right). The feed pawl is spring loaded so it can ride over the belt of ammo as it moves from the right to left, but locks down and holds the belt secure and pulls the belt in as it rides back left to right.
If you look at the bolt, you'll see "7.62" engraved near the top (to the left of the charging handle). Just to the left of that you'll see a part that looks to be a backwards, upside-down "L". That part pivots up and down along the left side of the bolt. It has a claw that extends down and grabs the rim of the next round in the belt. As the bolt recoils, it pulls the round out of the belt and and drops it down a "T-slot" into the bolt face. The bolt then slams home, chambering the round ready to fire again.
I'll have to explain the '42 later, but as you can probably see the track on the '42 is in the topcover whle the post is on the bolt.