This is a complex subject because of the number of scenarios Cubase supports. The simplest way to get started is to go into the pool and check the "Musical Mode" box.
In the project view, make sure you can see the info line. If you select the file you will be able to see that files settings in the info line. You can set things like the stretch algo.
On the track control panel is a little button that is probably set as a note that is yellowish. If you click that to a clock, the file will stay at the same time in the project, but it will scale speed with tempo based on the setting in the pool. If you set it back to the note, it will stay on the beat and scale to the tempo.
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