I know there’s a way to do this, I just can’t find it.
Copy a big chunk of sound from a long track. Select a shorter portion of a second track and Paste, but the paste cuts to the length of the second track selection. The length of the second track doesn’t change.
Also works the other way round:
Copy a short chunk of sound, select a longer portion of a second track and Paste, but the paste loops to the length of the second track selection.The length of the second track doesn’t change.