What do you need to know?
Multis are "just" a combination of several separate instruments and their settings, FX etc.
You could set all the instruments to the same MIDI channel to layer the sounds. Or keep them on different channels, as, for example "the set of instruments that I used in my song ABC".
https://www.groove3.com/browse/by-category/manufacturer/ik-multimediaThe MP2 video has a section on "Working with Multis".
You could try YouTube; you might try "Combis" instead of "Multis" as that was the name used in the older products.