Dan Moren, SixColors:

On macOS, we’ve long been able to add words to the system’s built-in dictionary, so they don’t come up every time we run check spelling.

And:

But this ability simply doesn’t exist on iOS. We’re told that if you override autocorrect a couple of times, the system should remember your correction, but in my experience, that “feature” is hit or miss. It’s also incredibly opaque and annoying as a user experience: the only way to have something done right is to fight with the system? Multiple times? Bizarre.

I agree with Dan here. I wish iOS offered a straightforward way to teach the autocorrect mechanism, to definitely mark a word as spelled correctly, and as a word you’d like added to the database of spellings to suggest when typing.

As is, predictive text is, indeed, opaque. It is always learning, and leans on machine learning, which is based on mathematical modeling, regression, and probabilities. While you do have the nuclear option (Settings > General > Reset > Reset Keyboard Dictionary), it’d be nice to be able to add to and edit your own private stash of words.