Buffets generally will have a waiter who will come take your orders for drinks, and yes, you can order almost any beverage. For non-alcoholic, almost every restaurant had a delicious watermelon lemonade with lemon-lime foam on the drink menu. For wine/beer/cocktails, there were a few exceptions, like wine over a certain amount per glass. As above poster mentioned, Biergarten is just one .5L beer, not a full liter or a flight.
Obviously, you don't get free refills on specialty drinks, but they typically would bring us a regular drink that did get refills like lemonade or coffee after the specialty drink if we asked, without any extra charge.