Its SZ, or sharp S. Eszett is how the german pronounciation of SZ would be written out. “Esset” would be used to encourage a group of people to eat a meal.
ß isnt used when you have a pair of s letters next to each other. Its most commonly used if you have long vowels beforehand. See “Trasse” vs “Straße”. Edit: “Phase” has also a long vowel but the “s” is pronounced less sharp…
Dreißig with “z” is wrong. If you cant be bothered to use “ß” on a non-german keyboard just using “ss” is completly fine. Because of the long vowels beforehand compared to zwanzig, vierzig, fünfzig, etc…
You got the number right, though! yay