Polar2100 wrote:
Auto-Correction is already on in my keyboard settings, but I'm still getting the red underline in Notes for lots of words that are, for example, technical terms in my industry, and in another example, an Ikea shopping list (their product names are all made-up, Swedish-sounding words).
I just need to be able to tell the Notes app not to flag certain words.
Hello and welcome, you are in a technical field with jargon (like most every other technical field), and legitimate jargon is being mis-detected as grammatical or spellings error in the Apple auto-corruption mechanisms, that using the usual little red-dotted underline, and you wish to not (unnecessarily) have those red-dotted underlines everywhere.
Okay, now that i have demonstrated that I understand your question fully, we can return to my reply.
Paraphrasing my previous reply: You can either correct the auto-corruption errors individually (that’ll be the double-quoted “literal” entry and that entry usually offered to the left of the choices shown), or can add jargon that looks like proper names into Contacts.
The quoted text use-this-literal-text-anyway selection is shown here: How to use Auto-Correction and predictive text on your iPhone, iPad, or iPod touch - Apple Support That’s the “In iOS 17 and later, Auto-Correction temporarily underlines words that it corrects. To quickly edit an auto-correction, tap the underlined word and choose an option from the pop-up menu.”
Why no simple “fix this?” button? Donno. Mac has a mechanism to explicitly insert these auto-corruption pacifications, err, corrections, while iPhone and iPad do not.
Given the jargon in the technical fields I deal with and the accruing spelling or grammatical errors that seemingly inevitably within auto-corrupt itself, I don’t get too invested in this correction process as I’m having to reset the spelling dictionary every year or so anyway, as the misspellings accumulate. Or I shut off auto-corrupt entirely.
Got feedback, let the folks at Apple know: Product Feedback - Apple
Por ejemplo: I’m just typing the “Donno” (yellow highlight) to intentionally trigger the autocorrection sequence, and that yellow “donno” triggers the quoted “donno” (red) in response to typing what autocorrection detects as an error at the keyboard then shows the training mechanism; the learn-this-spelling (red) option:

iPhone and iPad aren’t all that consistent at popping up literal spelling prompt immediately too, which can mean having to re-type a correct word again to trigger the “literal” correction sequence. Typing a period at the end of a word can cause autocorrupt to not show the literal “prompt” at the keyboard, and tapping on the red-dotted jargon doesn’t offer the accept-this literal prompt for instance.
TL;DR: autocorrupt can be arcane and flaky to train, and can sometimes be less than helpful, and the whole both-trained-and-mis-trained mess can sometimes need to be wholesale reset and restarted.