Is there any way to change the transparency of selected pixels?

in General
I'm trying to change the opacity of a group of pixels (make them transparent to see the background), is there any way to do this easily?
The only way I found is to select the color of each pixel and just manually redraw it with the desired opacity.
Example:

How to go from the top image to the bottom image without having to manually redraw everything
The only way I found is to select the color of each pixel and just manually redraw it with the desired opacity.
Example:

How to go from the top image to the bottom image without having to manually redraw everything
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