Agencies can contain many Datasets; each dataset has one Scraper; each time a scraper is run, it generates an Extraction.
Data lifecycle
There's a flowchart here which covers the end-to-end data submission process.
Scraping
The process for writing Data Scrapers is here. Writing a data scraper typically takes a handful of hours, but can be accelerated to just a few minutes by using common code to gather similar records.