The proxy server is a Golang service. It's where the partitioning logic is implemented, as I understood it. (Postgres seems to have native support for partitioning, along with some extensions available.)

They use the 'scatter-gather pattern' if the data is spread across many databases.

So many databases will be queried in parallel, and the result is combined at the app layer.

There's a performance tradeoff by querying many databases, but it happens rarely with Figma's query patterns.