Hi @James_Ellis,
default.qubit is a general simulator, not based on any particular physical device (more specifically, it is a pure-state qubit-model simulator). It simulates what a perfect, noise-free quantum computer would do.
The main difference with, say, an IBM machine is that the IBM machine will have some noise/imperfections which deviate from the theoretical ideal. As time goes by, and physical quantum computers improve, you can expect the real-world devices to track closer and closer to what the ideal simulators produce.