How surfaces stack: page, card, popover. Elevation in light comes from shadow, in dark from lightness.
A surface sits one step above the one it opens over. In light mode the steps are white and a shadow carries the lift; in dark mode each step is a little lighter, and the border does the rest.
--card#ffffff--popover#ffffff--card1.05:1 vs page--popover1.16:1 vs page