[ISO-morphism]

> what's going on under the hood

Rather simple methods are surprisingly effective [1]. There's sure to be more NN fanciness nowadays (like Apple's proposed NeuralHash), but I've used the algorithms described by [1] to great effect in the not-too-distant past. The HN discussion linked in that article is also worth a read.

[1] https://www.hackerfactor.com/blog/index.php?/archives/432-Lo...