Extractor service looks at the flow to find any files or other artifacts in it. Extractor-live is a storage space for "micro" extractions i.e. automatic extractions based on rules, as opposed to running one manually. Micro-extractions are for sending files to Data Enrichment on-the-fly.
There is no threshold-based auto-deletion going on. However there is a command to delete extractions: “scm extractions delete all” NOTE: that this could take a very long time to run.