Trying to answer that question can go in any number of different directions. Is there something more specific that you're researching or trying to troubleshoot? Knowing what you're dealing with might help us, help you, much more efficiently.
Owners manual lists all the fuses and relays in each box. For what each fuse powers, you would need the repair manuals which include schematics. The link is in my signature.
Here is a HD image of the Trailblazer-Envoy Under Hood Fuse Panel with correlated Fuse and Relay Types - Fuse Numbered Positions vs. their Locations on the Power Distribution Center Panel - and their Amperages:
Also... for DMM Testing purposes for Parasitic Draw Checks, etc... Have a look at this PDF on Fuse Types, Amps and Voltage Drops for ALL Types of Universal Automotive Blade Style Fuses: