My menu doesn't fit in the default screen in the mobile version

I've build this website: https://study-help.lu/wp/fr/page-dacceuil/

When I load it on my phone the menu just appears on the outside of the screen on the right. I know a bit of CSS but I don't know the difference where to modify CSS for desktop and mobile. I've used the Elementor builder. Any advice on how to fix this?

Thank you, Patrick