Elastic/OpenSearch have classically used BM25, but the latter recently added semantic search capabilities using neural embeddings in 2.4. Not sure about the former.
[1] https://opensearch.org/blog/opensearch-2-4-is-available-toda...
ES and OS are desperately slow because based on the lucene vector search index. A dedicated vector database like Qdrant will be always a better choice https://github.com/qdrant/qdrant