They work quite simply by drawing water through a siphon into the canister container. In the container, the water goes upward through a number of filter baskets that contain your filter media and then return to the tank using a pump at the top of the filter. I have a few pictures of one from when I cleaned it that may make the insides of a canister a bit less of a mystery. This is a 3 basket filter made by Rena called a XP3. It is a canister made for a fairly large tank. They also make smaller canisters for smaller tanks.
Bottom basket in place with 4 sponge filters stacked in it.
Second basket in place with bioballs in it.
All baskets in place, the top one contains pot scrubbers
Stack of baskets sitting on a towel on my desk
Assembled filter before moving into place and connecting to the hoses through those connection points that you can see on the top of the impeller housing.