Do other ingredients in brines penetrate the meat? I always thought, that only salt is able to penetrate deep into the meat.

However, there are many brine recipes, calling for additional falvors to be added.

How can they penetrate the meat, if only salt can penetrate the meat?

I also recently found the following question: How deeply will the flavors in a brine penetrate chicken?

The answer argues, that some flavours can indeed penetrate the meat, however food color can not.

So which one is it, is marinade a surface treatment only, or do other ingredients apart from salt also penetrate the meat, and if yes which ones?
Good Eats describes what happens in a brine very well

Essentially, the thing that penetrates the meat is water. Because water is a solvent it can carry things with it into the meat. Salt and sugar are the two most common ingredients in a brine. Salt is required and sugar dissolves really easily in water and adds complexity to the flavor profile. But you can add anything that will be carried by the water into the meat. If your brine isn't penetrating your meat past the surface you aren't doing it right.