Changing the Opacity is like mixing a cocktail with, say, 25% active layer and 75% all layers below. Assigning a blend mode is like shining a light or casting a shadow: The active layer infuses those behind it with life.
The modest brush tool is one of the deepest, most versatile tools in Photoshop. It's not only a terrific painting tool, but it is also invaluable for masking, as this video demonstrates.
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