Here are the steps to copy and paste in WSL Linux/bash terminals in Windows 10.
To copy text contents from a WSL window
To copy some text from WSL terminal window
Select the specific text to copy, then Press the Enter Key
Contents will be copied to the Windows clipboard. You may paste it in any text editor, email, web form, etc.
To paste text contents to a WSL window
To paste copied text in WSL terminal window
Mouse Right-Click
Note, whatever you have in your clipboard will be paster onto the terminal screen. If the clipboard is having multiline contents, all previous lines will get executed without any confirmation.
These steps are tested in Windows 10, with Ubuntu 20.04 WSL