fo+fb+fd

Hi guys,

When I was studying about laser vibrometers through Wikipedia, I noticed a vague problem.
Please take a look at the below pic which I have mentioned the question inside it.

The only way to sum 2 frequencies as I know is multiplexing them through a modulator, So how they got the fo+fb+fd just by a beam splitter or by the air/target?!!!
https://en.wikipedia.org/wiki/Laser_Doppler_vibrometer

Thanks

Does the environment acts as a modulator for the laser beam WHILE the object is steady and does not travel (so there is no Doppler effect...)?

Another important question regarding is :
suppose the target is moving (for instance a vibrating speaker which vibrates with the frequency of F1), so does the laser frequency (I call it F2) hitting to the speaker's cone will generate "F1+F2"??
Strange!