Query (→ GPT3 completion) → vector db lookups → GPT3 synthesis

The optional step two is used when the lookups are more closely related to an answer's latent space than the original query text. This approach is called HyDE (first published here: https://arxiv.org/abs/2212.10496).

The synthesis is also optional. You can essentially summarize your lookups or refine them or do whatever you want at this stage.

If you skipped steps 2 and 4, it's just a semantic search engine. If you skip step 2, you're either doing it for latency/performance reasons, or because the user query's embeddings are more similar to the docs in the vector db.