Interesting:

• https://imaginary.org/
• https://www.mathematik.de/

Educational sites and videos

• https://studyflix.de/mathematik
• https://www.studyhelp.de/online-lernen/mathe/
• https://www.youtube.com/c/MathebyDanielJung/playlists
• http://www.lehrer-online.de/subtraktion.php

Further links for exam preparation

• http://funktion.onlinemathe.de/
• http://www.abiturloesung.de (tasks and solutions from Bavaria and Hessen)