It’s definitely about vowel sounds, because we don’t say “a hour” or “a herb”.
Exactly, and Nvidia begins with an e sound.
And we say "a union" because it begins with a y sound and not because it begins with the letter u.
In the UK, people do say "a herb", but that's because we pronounce the h: https://en.wiktionary.org/wiki/herb#Pronunciation
I do, but that's because I pronounce the 'h' in 'herb'.
Kind of like how you see lots of people on Reddit saying "water isn't wet, it just makes other things wet". They heard someone else say it and it sounds smart so they repeat it, without spending 10 seconds to look it up in a dictionary.