Altium Shortcut to cycle through all PCB layers one by one
From the Altium documentation:

There are two approaches to interactively change layers during
routing:

Press the * key on the numeric keypad. Each press of that key will move you down to the next available signal layer.
Use the Ctrl+Shift+Wheel Scroll shortcut combination. Hold Ctrl+Shift, then scroll the mouse wheel forwards to move down through
the available signal layers, scroll the mouse wheel backwards to move
up through the available signal layers. Note that this shortcut can be
used at any time to change layers, if you are not currently routing
then this shortcut combination will step through all enabled layers.


This does not only work during routing, but you can use these methods any time.
While * brings you one layer down, Shift + * goes one layer up.