Image output destination is configurable on Mac. Cmd + Shift + 5, then it's under options in the bar. I don't recall all the options but it's at least file/clipboard.
⌘⇧5 (CMD + Shift + 5) doesn't do anything on my macOS 10.13.6 system, ⌘⇧⌃4 (CMD + Shift + Ctrl + 4) is the default shortcut to take a rectangle screenshot and store it in the clipboard.