I don't think they post schematic for the transformer
But generally, the primary coil receives 120Volt AC input from household electrical outlet
And the secondary coil produces 12Volt AC output for the low voltage lights.
The 'coil' is a coil of insulated wire wrapped around a metal core. Two coils set side by side create a transformer... when primary is energized, the secondary responds to the magnetic field caused by primary core... resulting in a current that flows on the secondary coil.
Output voltage on secondary coil is determined by the number of wraps and diameter of wire.
Some transformers offer a choice of secondary output... Kichler for example... you can select 12-13-14-15 Volt AC output
This is achieved by the way the secondary coil is wrapped.... for less voltage a wire is tapped at one point on the coil... for more voltage, a wire is tapped at another point on coil. This gives the user several wire choices to connect the lights.
If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7