Replace Keyboard Keys on XPS 15 9500 11/2020

Only you can see what the issue is.  If the keycap doesn't pop back on, you're almost certainly going to be faced with replacing the palmrest assembly - they keyboard is not a separately replaceable part (and Dell does not sell individual keycaps or components for keyboards).  There are some sources of keyboard parts, but this being a recent model, you likely won't find parts for the keyboard.

If you can't source third-party keycaps, this is the replacement part:

https://www.parts-people.com/index.php?action=item&id=32399

and here is the service manual:

https://www.dell.com/support/home/en-us/product-support/product/xps-15-9500-laptop/docs