Freighters

Freighters are private (AI control) ships which handle the meat-and-potatoes job of moving resources from where they've been harvested to where they're needed.

It should come as no surprise that the only component they need beyond the basic ship set is a Cargo Bay - Small, Standard, or Massive, as your technology permits. Like all private ships, they're limited to one weapon.

I am unaware of any distinction between "Small Freighter", "Medium Freighter", or "Large Freighter". It's possible the AI is more willing to move small amounts of resources (e.g. Nekros Stone) via "Small Freighters" on the assumption they're faster than the "Large Freighters", whereas it prefers the "Large Freighters" for moving bulk amounts of Steel around.