How can I teach my iPhone a new word?

I frequently change between three languages on the iPhone keyboard and want to add new words so the auto correction will help me in case I mistype them. I read that I have to tap red dotted words and then tap on “learn spelling” but all I get is “no replacement found”.

i don’t want to add hundreds of words to my shortcut list like ttyl and so on and I certainly don’t want to type the words in safari just so the iPhone recognizes them.

why is this basic feature missing (or just hidden somewhere??!)