There is no official hotkey to select all layers in Photoshop. You also cannot set one in the Keyboard Shortcuts menu. But it's still possible! Here's a tip on how to select all layers in Photoshop.

Selecting All Layers With the Shift Key

The easiest and most popular way to select all layers is to use the shift key.

1. Select the bottom layer in the Layers panel.
2. Hold the Shift key.
3. Select the top layer.

Selecting All Layers With Hotkeys

Here's a better way of doing this if you need to do this operation key a lot. Note that the hotkeys below use the comma and period keys.

Windows:

1. Press Alt+, to select the bottom layer.
2. Press Shift+Alt+. to select the top layer.

Macs:

1. Press Option+, to select the bottom layer.
2. Press Shift+Option+. to select the top layer.

You can also record this hotkey combination with a macro-keyboard. Many gaming keyboard have built-in macro keys which let you record a series of keystrokes. Here's an article comparing the best keyboard for Photoshop.