Use ctrl+shift+u then these character codes to get ä ö ü and ß

If you use a US keyboard need to regularly type non-US characters (like umlauts), the right way is through using a Compose key, which you can configure using a tool like tweaks.

But if you’re lazy (like me), and only need to type an occasoinal thing in German, this works for Ubuntu (at least in 20.04+):

press and release CTRL+SHIFT+u. then type the two-character code below, then press SPACE.

Example: CTRL+SHIFT+U, e4, SPACE. That gives you ä.