Control+F > type string > Alt+F3 > ESC > Right Arrow > line break
The important part being Control+Command+G to select all matches.
Once you've selected the text you're looking for, you can use the provided multiple cursors to do whatever text manipulation you want.
Protip: you can manually instantiate multiple cursors by using Command+click (or Control+click) to achieve similar results.
Using the Find - Replace tool, this can be accomplished in two different ways:
Click in the Replace field and press Ctrl + Enter to insert a newline (the field should resize but it doesn't, so it is hard to see the newline inserted).
Inside the Find - Replace tool, activate the S&R regex mode (first icon on the left .*, keyboard shortcut is Alt + Ctrl/Cmd + R to activate/deactivate it).
Type \n in the Replace field wherever you want to insert a newline.
Both solutions also work if you want to find newlines, just do it in the Find field.