The user manual has fairly good diagrams.
Get user manual here:
http://www.mackie.com/products/1604vlz3/pdf/1604VLZ3_OM.pdf
On page 34 is the BLOCK layout which has
everything you need to know, once you figure how to read it!!!
The Triangle symbols are operational amp/analog summing amps.
By looking at the jack symbols you can tell if they are mono or TRS.
A lot of the TRS ones will show a TRS three lead jack but have a resistor to ground. These can be used as mono's.
The TRS (stands for Tip-Ring-Sleeve) connectors are used in two ways. One is as a balanced input where both the signal and its inverse is used. A balanced line is far more noise free and ground loop free than an unbalanced line.
Another use of the TRS is for external effect insertion where the sleeve is ground and the tip is the signal SENT to the external effect box and the ring is the RETURNed signal from the effect box.
SOMETIMES you may have to make your own cables to get the signals where you want them on the connectors.
HINT: Use XLR connections wherever you can. More rugged, balanced and noise free- ground loop free.