Finnish is an interesting language. After I looked up the etymology of 'food' - late old english foda, originally germanic, I can see how the replacement of consonants (very common in indo-european language evolution), could arrive at ruoka, and kuuk and kosmetiikka and hygienia provide some clues, but the rest is, well, Finnish :D.
Finnish is an interesting language as it is "new" in the sense that the first written form of it was compiled about 200 years ago by one person. This became the "book language" that is taught in schools and it is rule based with very few exceptions. It is like math. Now though, there are also "loan words" that come into it, as you noticed above with kosmetikka and hygenia. There are also mani other ones like bussi and taksi. It also loans words from Swedish and Russian.