How to systematically join snapshots

[gwenwindflower]

This is a tricky technique! It's best practice to snapshot source data so you can capture everything, but we can then find ourselves needing to bring many (and in some cases many many) snapshots together to craft the entities and metrics we want. How do we do this correctly?

@lbenezra-FA has written a guide! transferred from this archived discussion in the archived technical writing repo