Layers in Photoshop:
You can think of Layers as flat panes of see-through glass, all separate and stacked one on top of the other.
Each layer contains separate content - the images, text, or objects that make up a layered file in Photoshop.
They let you move, edit, and work with content on one layer without affecting content on other layers.
Layers are arranged in a stack in the Layers panel, which is usually located in the bottom right of the work area.
If the Layers panel is not visible, choose Window > Layers.
The Layer Panel:
Opacity slider of the selected layer affects the transparency or the visibility of the entire selected layer.
Fill slider changes the visibility of only the contents of the layer.
The layer with a mini square in the corner = its a shape layer.
The layer thumbnail with a capital ‘T’ = a text layer. Photoshop automatically creates a new layer with this thumbnail for text.
The thumbnail of a layer without any shapes or symbol means that the layer is a digital artwork (i.e. a png, jpg or jpeg file). The eye symbol beside a layer is a toggle to hide/unhide a layer.
The vertical clipping symbol in a layer means it’s a layer with a mask applied to it.
If you click the eye icon to the left of a layer you can "hide" its content. Click again in the same spot to make the content "visible" again. This is a useful way to remind yourself what’s on a particular layer.
A layer must be selected in order to make changes to it. In the Layers panel, click once to the right of a layer name to select that layer. To add more layers to your selection, hold Control (Windows) or Command (macOS) as you click other layers.
Below: I've put together 4 separate layers to create the finished artwork on the left:
Video Resources on Working With Layers in Photoshop:
How to use Layers and Layer Mask?
How to Open Images as Layers in Photoshop