This is a dumb question, and I promise I did my due diligence trying to answer it myself first but my googling has failed. What controls how the data is handled at the target? If I set the tap to full replication, what dictates whether the target will do an upsert or an append?

Each target is different slightly I'd look at the target code. Generally it's if a PK exists it's an upsert. If there's no PK then you append 🤷

Ah, that makes sense. I don't think this convention was in the documentation anywhere, or else I missed it. Thanks.

🤷 my answer isn't perfect as it depends on the target I think

This is indeed a really good question (and a good answer). Learning Meltano, I was very surprised by how much of the behavior was defined by the taps/targets. Depending on how easy we want it to be to get going with meltano, we should really find some way to surface expected/common behavior and if a tap/target conforms to this.