the electromagnetic field is linear — that means that if you add two signals of any kind (whether this be digital, analog, or by means of having two separate transmitters) this does not result in there being signals at other frequencies than you started with

do note that just like any approach to modulation, there's a nonzero bandwidth, a range of frequencies that the signal occupies, once you start doing anything more than the carrier

@NicHartley If you're doing QAM, then you are increasing the bandwidth. Your center frequency will still remain 2.4GHz*, but it will now occupy 2.4GHz +/- 100kHz. The band width depends on how you modulate the signal.

there's lots of different ways in which it appears but in general it is always — there is some range of occupied/permitted/relevant frequencies [f_1 … f_2] and f_2 - f_1 is the bandwidth

and the way in which it is relevant to modulation is that:

A theoretically perfect unmodulated carrier has zero bandwidth, but also can transmit zero information. Any way you modulate it increases the bandwidth.

The math is nontrivial and the practical engineering answer is "look it up for the modulation you're using".

And it depends on a bunch of parameters

I think you might benefit from reading through some introductory text on digital communications — there's a lot of things about modulation and fundamental communications concepts that are all tied together