> Or, it could be the case that only GM makes GM-compatible brake pads. In which case they would have a monopoly in that market. Not because it’s a monopoly on their brand of brake pads, but because it’s a monopoly on any brand of brake pads compatible with that brand of cars – which is something else entirely.

> Notice that they don’t even have to be the same company.

As a specific example (and note, that you also don’t have to be the literal sole supplier to have a legal monopoly under US antitrust law), the market Microsoft was found to have monopolized in their big antitrust case was the market for operating systems for IBM-compatible personal computers.