How do I merge all layers in Photoshop?

To merge all visible layers in your Photoshop document into a single layer, you have a couple of easy methods.

Method 1: Using the Layers Panel

Select all layers: Click on the top layer in your Layers panel. Then, hold down the Shift key and click on the bottom layer. This will select all layers between your first and last click, including them.
Right-click (or Control+click on Mac) on any of the selected layers.
Choose "Merge Layers" from the context menu. This command combines all selected layers into one.

This method is even faster because it doesn't require you to select all layers. It merges all visible layers into a single layer.

Press Shift + Ctrl + E (Windows) or Shift + Cmd + E (Mac). Photoshop will merge all visible layers into one new layer.

Visible Layers Only: Both methods only merge layers that are visible (i.e., have the eye icon enabled in the Layers panel). If a layer is hidden, it won't be included in the merge.
Rasterization: Merging layers essentially rasterizes them. This means that any vector layers (like text or shapes) will be converted to pixel-based images. You will no longer be able to edit them as vectors. Consider duplicating layers before merging if you need to retain editable versions.
Alternatives: Instead of merging, you can also flatten the image (Layer > Flatten Image). Flattening also merges all layers, but discards any hidden layers and fills any transparent areas with white. This is a destructive process. Merging is preferable if you want to keep hidden layers for later, and allows for easier recovery if a mistake is made (simply Undo the merge).