Fastest way to import billions of documents?

There isn't a "native" way, really. There are filebeat and logstash, both of which are things actively worked on by Elastic folks that stick files into Elasticsearch but they are both more organized around many servers loading into one Elasticsearch so they only go with a single bulk.

Elasticsearch doesn't support any kind of streaming loads like would be required to make a supermagic curl command work. We've talked about it but never done it.