A mixture of threads and processes that can be used to match processors, disk I/O, and network interfaces.
A very long time ago, there was once a feature called "Data Blades" which tanked a commercial database vendor. A badly behaving blade could bring down the entire database. Most anyone who has been working on databases for a few decades remembers this and makes a point of either not introducing these sorts of features or making use of processes over threads.
I have not looked at the code referenced in the mentioned project, but thus far I haven't seen a model that could craft a complete SQL parser on its own.
There are a number of problems, and design decisions, that a developer decides on when writing a database that I don't see any current models… just because you have the ingredients does not mean that the stew is edible.