Accented letters in Ubuntu 20.04
Solution 1:
You're looking for so called "dead keys". Those are keys that don't print a character (on the first try), but influence the next character. For example, if you press ', it doesn't have an immediate effect, the key seems "dead". But if you press e directly afterward, you get é.
In Ubuntu, you can change the keyboard layout you want to use. Some layouts include dead keys, others don't. So if you want to use dead keys, just choose a keyboard layout that supports them.