Multi-cursor selection in WebStorm:
Select next occurrence of the current word or selection: Ctrl+G on macOS and Alt+J on Windows and Linux
Select all occurrences: Ctrl+Cmd+G / Shift+Ctrl+Alt+J
To add or remove multiple carets, you can also press and hold the Alt key, then click at the next location of the caret. Alternatively, to add multiple carets, you can press Alt/Ctrl twice, and then without releasing it, press the Up or Down arrow keys. You can also make a multiple selection by pressing Shift+Alt and dragging the caret.
To delete all carets, press the Escape key.