why we do multiple aliasing of high order harmonics
Aliasing is more like circular wrapping around rather than just mirroring.  Frequencies wrap circularly between -Fs/2 to Fs/2, an 800 Hz range from -400 to 400 at a sample rate of 800 sps.
120 is inside -Fs/2 to Fs/2, so doesn't need to wrap around at all.
But 480 wraps 80 past 400, and shows up at -320, which looks like 320 if you don't care about phase.
And 1120 wraps all the way around the circles range of 800 Hz, plus another 320, so it shows up at 320.
12000 Hz (12 kHz) would wrap around 15 times and show up identical to DC or 0 Hz.