German characters on US-International keyboard layout

AltGr+s. You could get help in Windows Keyboard Layouts, in particular at United States-International (with AltGr pressed):

How do I enter the ß (sharp s)?

ß Right Alts Esszett (sz ligature)

Source Help with Microsoft Keyboards US-International | School of Languages, Cultures, and Race | Washington State University

Personally i use this layout https://eurkey.steffen.bruentjen.eu/

It's basically the US layout with all of the umlauts (and anything else) you might need.

äöüß would be AltGr + a/o/u/s.