Replying to my own comment: I found a 2021 paper that goes over all the different technologies being considered, and the wide variety is frankly staggering. Table 10 lists ten different principles under consideration (fluorescence, spectroscopy, etc.) with their various pros/cons. Not ELI5, but as close as you might get:

https://iopscience.iop.org/article/10.1088/2516-1091/abe6f8#...

Seems like spectroscopy seems particularly promising, and according to the article, that's the approach Apple is taking. (Makes sense for a watch.)

And from the paper, the challenges with spectroscopy are signal interference -- scattering, water absorption, and distinguishing between glucose and other sugar components. So it ultimately sounds like a fiendishly complex signal processing problem. Which is exactly the kind of problem that is very very hard yet maybe not impossible.