Not every buy is a full proposal competition. Agencies are told to use simplified acquisition procedures to the maximum extent practicable for all purchases of supplies or services not exceeding the simplified acquisition threshold 1.
Another channel is the schedules program. The FAR notes that the Federal Supply Schedule program is also known as the GSA Schedules Program or the Multiple Award Schedule Program 2.
Worked example. A small training company sees the same kind of course bought by several agencies through schedule orders it never saw on SAM.gov. It decides to research what holding a schedule contract would take before chasing more open-market notices.
Indented paragraphs are invented worked examples. Tool links open Bids2Bank and need a free account.