ELI5 Deserts - was the Sahara really once lush green forests?

Did you know that some of today's deserts were once seas, lakes, or lush forests? How do oases exist in such arid regions? Why does the US have huge cities in the desert? Why did people settle there? How is the Sahara linked to the thriving of the Amazon rainforest?
... we explain like I'm five
Thank you to the r/explainlikeimfive community and in particular the following users whose questions and comments formed the basis of this discussion: dreadmcyt, murky-entertainer-81, toddwhiskey, teelmetheworld, fehnboi, exponentials, a_saddler and chel_of_the_sea
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