The majority of English speakers are not proficient in the International Phonetic alphabet or any other phonetic transcription system outside their own orthography. However, we often feel the need to write something in a way that is more indicative of its pronunciation than traditional orthography. This is especially the case with vowels, where many speakers turn to respelling methods with digraphs and trigraphs to represent each vowel phoneme.
Some of the standard respelling graphemes are very logical - like ⟨ee⟩ for the FLEECE vowel and ⟨oy⟩ or ⟨oi⟩ for the CHOICE vowel - because those digraphs often represent those phonemes in standard orthography. However, other respelling patterns that have emerged exist in hardly any words in the orthography, and thus seem extremely odd to me. Many of these utilize the letter ⟨h⟩ despite no resemblance of the sounds they represent to the /h/ phoneme (the same can be said for consonant digraphs like ⟨ch⟩, ⟨sh⟩, ⟨ph⟩, ⟨th⟩, though these are a bit more understandable) For example, English speakers use spelling patterns like ⟨uh⟩, ⟨ah⟩ ⟨ih⟩, ⟨eh⟩, ⟨oh⟩, and ⟨ohw⟩ in respelling transcription.
The existence of these transcription choices can be observed in the pronunciation YouTube channel English Hacks - Feel English Like A Native. For a variety of poorly-explained reasons, the creator, Josh, chooses to use his own phonetic transcription system rather than utilize or modify an established system like the IPA. His system includes ⟨uh⟩ for the STRUT and COMMA vowels, ⟨ih⟩ for KIT, ⟨eh⟩ for DRESS and SQUARE, ⟨uuh⟩ for FOOT (an odd choice), and ⟨ah⟩ for TRAP. Personally, I think of ⟨ah⟩ as the LOT/FATHER vowel, but Josh's use of the cot-caught merger (he writes ⟨aw⟩ for both of those vowels) and my lack of it sort of explains that.
It seems probable that interjection words like ah (fear and surprise), oh (realization), uh (hesitation), huh (confusion), and possibly meh (indifference) have something to do with these respelling patterns. However, I still don't know why those interjections are spelled that way, and they don't explain the existence of ⟨ih⟩. In addition, eh (stereotypical Canadian yeah/huh) is a weird one since we usually think of that word as having a FACE vowel but use that letter pattern to denote DRESS vowels.
How and why did these digraphs originate? Why is the letter ⟨h⟩ involved in all of them? Do speakers of other languages use any similar spelling patterns?