Learn Finnish: Noutopöydässä voit yhdistellä annoksia vaikka miten. - At the buffet, you can combine the portions any way at all.

They mean exactly the same thing: "any way at all" or "however you like". Using vaikka before a question word (like miten, kuka, or missä) is simply a more colloquial and expressive alternative to using the tahansa structure that you learned at earlier levels.

What is the difference between yhdistää and the verb used here, yhdistellä?

yhdistellä is a frequentative verb derived from yhdistää (to combine or connect). The -ella/-ellä suffix changes the meaning to doing something repeatedly, casually, or in many different ways. Here, it perfectly captures the idea of "mixing and matching" at a buffet.

Why is annoksia in the partitive plural here?

It is in the partitive plural because the action is open-ended and indefinite. You are combining an unspecified number of portions, and the action doesn't have a final, finished endpoint. Furthermore, frequentative verbs like yhdistellä inherently describe continuous or scattered actions, which naturally trigger partitive objects.

Where does the word noutopöytä (buffet) come from?

It is a descriptive compound word made of nouto (fetching or pick-up) and pöytä (table). It literally translates to a "fetching table"!