What is the difference between popover menu, drop-down menu & fly-out menu?

What is the difference between popover menu, drop-down menu & fly-out menu? Are they same in functionality or do they vary?

If no, why so many variations?


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Popover menu drop-down menu

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Fly-out menu enter image description here

For reference:
https://experience.sap.com/fiori-design-web/quickview/ https://lightningdesignsystem.com/components/popovers/ https://getbootstrap.com/docs/4.0/components/dropdowns/

Note: These are just for references!!

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