Can I ask you which alternatives exist at the layer Lucene occupies?

I went looking around last year and couldn’t really find many options, but I might have been looking in the wrong places.

For Vector Search the top 2 are: Meta’s FAISS and (my) Unum’s USearch. Lucene powers Elastic, Solr, MongoDB Atlas, AWS OpenSearch, Azure Cognitive Search. USearch powers ClickHouse, DuckDB, YugaByte, TiDB, ScyllaDB, MemGraph, KuzuDB, Lantern, and a few big closed source names that don’t mention it, as far as I know. FAISS has the highest usage among Python developers, but if you are indexing large collections you should consider alternatives.