Also, am I correctly assuming that:

.write_deltalake()

locally, it seems it does save / write each part over time separaetly, but on S3, it will upload only at the end?

The behavior should be the same whether doing a write locally or on S3

Could you maybe elaborate, when the writing happens? How does it decide how to split the parquet files?

Daft writes each partition to disk/cloud, but also respects an estimated target file size (so it will try to split up partitions into multiple files if they get too large)