Since I don't know about current versions of MongoDB, and am genuinely interested, how does Mongo know how much memory to use as an example?

Typically with PostgreSQL, you have to expand the defaults when running on modern server class hardware, so the engine uses enough RAM. Simply letting it use all RAM available can be problematic, since the DB engine and OS have to fight for cache, etc.

Java based systems like Elastic might still need JVM tuning, especially with a distributed platform like ES, which might be run in a large number of small machines, a smaller number of large machines, or containers.

You are exactly right. You must adjust the java heap size on Elasticsearch.