• Command + E (Mac) | Control + E (Win) will merge the selected layer with the layer below.
• Command + E (Mac) | Control + E (Win) will merge the selected layers when multiple layers are selected.
• Command + Shift + E (Mac) | Control + Shift + E (Win) will merge visible layers.
• Command + Option + E (Mac) | Control + Alt + E (Win) with multiple layers selected creates a new layer and pastes a “flattened” version of the selected layers on it.
• Command + Option + Shift + E (Mac) | Control + Alt + Shift + E (Win) creates a new layer and pasted a “flattened” version of all (visible) layers on it.