I went years without knowing how useful the Alt key was, it’s not well documented but I can guarantee that once you know about it and give it a little practice you won’t be able to live without it.
While I’m filing this under SSMS Tips and Tricks but it works equally in Visual Studio, VS Code, Azure Data Studio and even Notepad ++ (but not plain old vanilla notepad). It’s worth a test in other places too.
As a quick note, it does not work quite the same way in Visual Studio Code or Azure Data Studio as in Visual Studio or SSMS: Alt+Shift+Down arrow copies the current row and pastes it in the row below. Holding Alt and Shift while using the mouse does work, but if you’re big on keyboard commands, you’ll be a bit disappointed.