Yes, the confusion is that when there is only one resonant frequency in the system,
its the natural frequency too, they are the same.
But if there are multiple, the natural frequency might not be the lowest resonant frequency.
Probably is, but possibly isn’t.
The natural frequency is the frequency it resonates when stimulated with a broad spectrum, eg from an impulse… … which in mechanical terms means hitting it with a hammer, for example. (or the edges of square waves.)