It is not a B

In German orthography, the  grapheme ß, called  Eszett (IPA: [ɛsˈtsɛt]) or scharfes S (IPA: [ˈʃaɐ̯fəs ˈʔɛs], [ˈʃaːfəs ˈʔɛs], lit. "sharp S"), represents the [s] phoneme in Standard German, specifically when following long vowels and diphthongs, while ss is used after short vowels. The name Eszett combines the names of the letters of s (Es) and z (Zett) in German. The character's Unicode names in English are sharp s[1] and eszett.[1]

It originates as the sz digraph as used in Old High German and Middle High German orthography, represented as a ligature of long s and blackletter typography (ſʒ), which became conflated with the ligature for long s and round s (ſs) used in Roman type.

In other countries than Germany and Austria, the letter is not marked on the keyboard, but a combination of other keys can produce it. Often, the letter is input using a modifier and the s key. The details of the keyboard layout depend on the input language and operating system, such as Ctrl+Alt+s, on some keyboards such as US-International also AltGr+s in Microsoft Windows or Option+s on the US, US-Extended, and UK keyboards in macOS. In Windows, one can also use alt code 0223.

Some modern virtual keyboards show ß when the user presses and holds the s key.

Wikipedia