ELI5 Blood Oxygen - why are pulse oximeters suddenly so popular?

How do pulse oximeters work? Why have they become popular recently? Can they be used to detect Covid-19?

... We explain like i'm five!

Thank you to the r/explainlikeimfive community as always and in particular the following users whose questions and comments formed the basis of this discussion: cutelyn22, internetboyfriend666, scubasteave2001, scarynut, bravo_bravos & rauillindion

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