Welcome to PSG!
Say that you want to copy the info from the green channel of a layered image to a new layer.
ALT+CRTL+2 copies the greyscale info from the green channel (the numbers are shown in the channels palette. For composite, it is CTRL+ALT+~) another solution is to CTRL+click the channel
CTRL+D to deselect (we'll recover the selection later)
CTRL+SHIFT+ALT+N creates a new layer without the menu
D to default the foreground color to black and background color to white
ALT+Delete fills the layer with foreground color (here, black)
CTRL+SHIFT+D recovers selection
CTRL+Delete fills with background color (white)
You have now copied a channel into a layer.
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If you want to copy an existing channel into a layer mask:
Click on the layer that you want to add mask to.
ALT+ click on the add layer mask icon to create a hide all layer mask (black)
ALT+clicking the layer mask thumbnail to target it (image turns black)
CTRL+click the channel that you want to copy the luminosity from
CTRL+Delete to fill the selection with foreground (white)
You have copied a channel into a layer mask.
For all the keyboard shortcuts, see:
http://user.fundy.net/morris/photoshop1.shtml