> maintenance_work_mem

That kills the indexing process, you cannot let it run with limited amount of memory.

> How do you think a B+tree gets updated?

In a B+Tree, you need to touch log H of the pages. In HNSW graph - you need to touch literally thousands of vectors once your graph gets big enough.

> That kills the indexing process, you cannot let it run with limited amount of memory.

Considering the default value is 64 MB, it’s already throttled quite a bit.