Taking a screenshot on Samsung Galaxy S21 can be done quickly by using the volume-down button and the power button simultaneously for 3-4 seconds. You can also use the recent screen capture feature by swiping left from the right edge of the screen. To edit screenshots without an image editor app, you can use Samsung's built-in screenshot editor, which allows you to highlight specific parts and add notes.
To access the screenshot editor on Samsung devices, go to the Screenshots folder in the Gallery app, tap the three dots at the top right corner, and select 'Edit'. You can then draw on your screenshot, add text, and even crop it to get exactly what you need. The edit feature is available on most Samsung models, including the S21 series.