I found this quite useful paper, which describes the design of a commercial 802.15.4 transceiver. It says:
"The modulated and spread I/Q baseband signals are applied to the digital-to-analog converters (DAC), whose outputs are low pass-filtered and up-converted directly to RF by a single-sideband modulator."
So my understanding now is that the 2 MHz modulated baseband sinewave is simplify multiplied together with 2.4 GHz carrier signal, effectively modulating the amplitude of the 2.4 GHz sine wave. Can someone confirm that this is correct?
Update: continuing to investigate, this article has more details about the math of singnal up- and down-conversion in form of formulas and pictures. That is basically what I was looking for.