Hello Felix!
To create internal pages, you have a few options:
OPTION 1
You can create simple designs in WordPress using a free plugin like Page Builder by SiteOrigin
Video tutorial: https://www.youtube.com/watch?v=mfrEMZw899M
OPTION 2
You can create simple & advanced designs in Themes Generator using the Extra Page Template block.
Video tutorial: https://www.youtube.com/watch?v=gvxocH-0nxw

TO MANAGE THE MENUS
You can assign pages to menus using WordPress control panel under Appearance > Menus.
Video tutorial: https://www.youtube.com/watch?v=OUxNB9rvGsk
I hope it helps!