We don't have a proper design document (but I certainly think we should have one). I will try to offer a high level summary:

At the heart of Typesense is a `token => documents` inverted index backed by an Adapative Radix Tree (https://db.in.tum.de/~leis/papers/ART.pdf), which is a memory-efficient implementation of the Trie data structure. ART allows us to do fast fuzzy searches on a query.

All indices are stored in-memory, while the documents are stored on disk on RocksDB. All underlying data structures were carefully designed, benchmarked and optimized to exploit cache locality and utilize all cores efficiently.