Mackie Onyx are not powered mixers, so you can only use active speakers. The exact configuration of the mixer depends on the model (which you fail to mention).
On the models I looked at, the XLR outputs can be switched between +4dBu and mic level and (unsurprisingly) have separate output drivers because of that.
This is easy enough to see in the block diagram included in the bog standard user manual for the mixer. Have you bothered looking at the documentation? Even if you for some reason do not have it any more, it can be downloaded from the Mackie website.
If you are more curious or technically inclined, even the full circuit diagrams can be downloaded from there. There really is no reason to go hunting on external websites for information that Mackie willingly shares.