I have been on the hunt for what exact shortcut the controller hides from us. But there is a steam menu (controllers, advance, show more, at the button) that shows them!
Quickly want to change your volume? Try steam key + left joystick!
Sadly you can not change anything here, altought the menu shows you you can, nothing will be saved after you leave a menu. (I'm not a fan to put the left mouse button on the right button)
A few of them I manually found before written out, also for the people without images:
hold A + R1 + Steam
To use Puck wireless mode right
hold A + L1 + Steam
Start Puck wireless mode left
hold B + R1 + Steam Start Bluetooth mode
When on (solid white)
Steam+Y Turn off quickly
Hol steam (10s) Turn off
Steam + left joystick up/down Change volume level
R4 + R5 + L4 + L5 + Steam Backpack mode (ignore buttons until this combo is used again or USB)
Steam + Right trackpad/joystick Mouse movement
Steam + X On screen keyboard
Steam + R1 Take screenshot
Steam + Menu ALT+TAB
Steam + D-pad right Enter key
I skimmed the post so maybe I missed this but this is the guide button chords (or something like that) in the advanced controller settings, I have a few extra things on dpad like enter escape tab and adding a video marker. Honestly only use case I have for those specifically is to be a mode shift with extra steps cus one game I have (Jump space) I have exit and interact as the same button which leads to some funkiness in particular situation
Apart from that being able to adjust volume and a few other stuff on my keyboard without having to reach over is nice (oh my god walle was right)