Why QAM builds a signal from two carriers at the same frequency that are 90 degrees apart in phase, the in-phase (I) and quadrature (Q) components. Each carrier's amplitude is varied independently, and their sum produces a single signal whose amplitude and phase together encode several bits per symbol. Plotting the possible I and Q amplitude pairs gives the familiar constellation diagram, for example 16 points for 16-QAM carrying 4 bits per symbol.
Watch out The choice about shifting carrier and modulation phase describes the phasing method of generating SSB, which also uses 90 degree phase shifts but produces a single sideband analog signal, not a multi-level digital symbol scheme. The compression answers confuse modulation with data reduction; QAM adds no compression.
Quadrature = 90 degrees. Two carriers, I and Q, amplitudes varied, and the constellation dots show the result.