German has 4 special characters that you need to be aware of, if you’re typesetting a German document. The 3 umlauts ä, ö, ü and the special ligature ß.
The umlauts are like regular characters. They come in lower case and upper case variants. ä, Ä, ö, Ö, ü and Ü.
The ß (sz ligature) is a special kind of character. First, there is no upper case variant of it. (At least not until recently, but it’s still a debated topic) And second, you should not confuse it with an upper case B or even the Greek beta character β! Those are completely different characters.
There are also some special rules regarding upper-casing the ß. Since there is no upper case ß, the ß becomes SS. But not every SS will become a ß when down-cased. (e.g. upper-casing “Straße” becomes “STRASSE”. But down-casing “WASSER” becomes “Wasser”).