almost always, a/c from the wall goes in the bottom and out the top of transformers but not always.
no matter which way you hook it up with the black on one terminal and the white on another, a/c (alternating current) does just that, it alternates. when measuring for ac with your handy dandy multimeter, you always test both legs of the outlet or put your meter probes into both outlet holes. screwy reading can be obtained if you put your black probe in the third outlet or earth ground hole and the red lead into one of the other outlet holes because you are just measuring one leg or half the power of the wall.
ac alternates 60 times a second or 60 hertz. bridge rectifiers chop the ac in half to start getting it ready for other voltages needed for other game requirements.
transformers you can tap in at different points in the long windings to get different voltages. 6.3 to run some coin door lights. 12 volts for audio, 25 volts, 36 volts, etc., etc.
the windings have a coating on them to keep the electrons flowing down the full "pipe". think of going to home depot to buy a plumbing pipe to fix your toilet but wires come already "full". taps cut into the windings to release the electron flow for the coin door lights. a short across the coating renders the transformer inoperable. I've tried semi successfully to re-wind a monitor yoke although it buzzed like a bastard. found a working yoke but that's another story.
transformers generally don't change a/c to d/c (direct current). they generally change the ac or dc to another form of ac or dc. so if you see a transformer in a circuit, you kind of know whats going on there but always check your schematics if you're looking at a monitor chassis or anything else trying to find a fault.
always make sure your earth grounds are clean and tight or the electron flow will try and find a quicker path to ground and you don't want it doing that THROUGH you. ZZZZZZZZZ, ouch ! dumbass.
phone rang and broke my train of thought.
good luck