The last ability of Endless Sands returns only creature cards exiled by that card. If Endless Sands leaves the battlefield before you activate that ability, the exiled creatures are lost in the dunes forever.

If you exile a creature that isn’t a creature card with the second ability of Endless Sands (such as a token creature or a land that has become a creature), the third ability won’t return that token or card from exile.

Desert is a land subtype with no special meaning. It doesn’t grant the land an intrinsic mana ability. Other cards may care about which lands are Deserts.