how does water from the tank go into the container?
An overflow. This is either something built into the tank (predrilled) or hangs on the back (HOB
overflow) Overflow box's are also sometimes refered to as prefilters. The U tube overflows are better IMO
than the C style. The U tube ones use round tubes shaped like U's to get the water up and over the edge of the tank.
The intank part of the overflow has fingers our fringes at the top. Water sifts thru these and then into the intank box. from there syphon pulls the water up and over the tank thru the U tube, the outside tank portion of the overflow is split into two chambers. with a wall going approx 1/2 way up the overflow box. The first chamber is where water exits the U tube and then cascades over into the other box. This keeps both ends of teh U tube constantly under water even in the event of a power outage. Thus the syphon will not break simply by unplugging the pump. In the other chamber there is a drain line that drains down to the sump.
water is pumped UP from the sump via a pump and plumbing and is returned to your tank.
Your 3 sectioned sump is actually more fancy than it needs to be. For example my sump is 1 chamber. No partions, no baffles. Its just a 30 gal
long tank where water enteres on the left and the pump takes the water out on the right. In the sump i have my heaters and some large live rocks to increase bio filtration. If you had a skimmer you would have it in the sump aswell if you wanted.
If you look thru the photo gallery at the Members tanks and setups section you should see a fair number of sump setups. Below is a link to my sump. You can see part of the instank and out of tank overflow box's and the u tubes.