How to use screen capture UI standalone, to edit existing images?
By default, since macOS Mojave, when taking a screen shot a floating thumbnail briefly appears in the lower right-hand corner of the Desktop, which one can click on to edit the screen shot.

The editor which appears, because by default Show Floating Thumbnail is checked in the Options for taking a screen shot, can be used on an image file or screen shot afterwards by selecting the image file in Finder and then right-click on the image and select: Quick Actions > Markup

You can also, after selecting the image file, press the spacebar to bring up a quicklook view and click the Markup button.