Those are fair questions. At least on my end, I'm using a pretty direct analogy to a car engine.
A combo engine is a combination of cards that work together to create a cycle. The engine parts are usually interchangable, as they are just there for the purposes of resource conversation. An engine needs some sort of fuel to power its interactions. It must have a continuous fuel source on one end, and a source of (hopefully) game winning output on the other end.
The fuel source in the familiar deck is high octane mana (bouncelands). The combination of cards working together to create a cycle that converts that fuel into output are:
cloud of faeries,
archaeomancer (or
mnemonic wall) and
ghostly flicker. Sometime it may need slightly more cards depending on the game state (the namesake familiars, which act like a supercharger). When those pieces are operating together, it produces an infinite cycle. The output from each cycling of the engine is enough to keep the engine going, but because we are using high octain fuel (bouncelands) and a supercharger of sorts (the familiars) we produce excess output with each cycling of the engine. We can than leverage our hundred million mana excess, by putting it into another engine piece
capable of ending the game, or recalibrating the engine away from mana production to golem production.
You could use terms like cogs, but at the end of the day I think we are all searching around to describe that sort of mechanical relationship of moving parts working together to process fuel and produce an output.
All magic decks are systems, but most of them are much more linear than the above. They consume a resource, which they use to exert an action, which results in waste. Some of them may use resources more efficently than others, or use their resources to make more impactful actions, or use resources to disrupt an opponents use of resources, but its still a linear line of consume->take->waste.
Even the aggro-combo decks are more linear, and though they focus on drawing together a combination of cards to end the game, they are not really a combo
engine. If might be a better analogy to think of those decks like combining explosive components, the threat of which you than try to leverage. This is part of why I am such a proponent of burst damage in aggro archetypes, as it adds a combo feel to them, even if we aren't setting up a true engine.
Put speaking of creature based engine pieces: