Learn English – What would be the onomatopoeia for “spit”

I'd say that your German colleagues are mishearing the English pronunciations.

The German letter ü makes the sound [y], which does not occur in English.

The words loose, poodle, food, and most other words with oo have the vowel [u], which is usually spelled u or uh in German. Historically this is a long /o/ sound that was written with "oo", the pronunciation of which has shifted to [u] as a result of the Great Vowel Shift.

Some words with oo have instead the vowel [ʊ]: good, hood, book. There is no rule that predicts which words have this pronunciation, so you have to memorize it. The [ʊ] sound occurs in German as an allophone of /u/ in closed syllables. The vowel [ʊ] is shorter, more lax, and slightly centralized relative to [u]. This sound also tends to come from an older long /o/, though the reasons for this split are complicated and obscure.

A very small number of words with oo are pronounced with an [o] vowel: door, floor. These words always end in r, because the final r colors the preceding vowel. This is the same sound that is spelled o or oh in German.

The ī symbol used in Google's rendition of hypotension is a simplified form commonly used in dictionaries. The bar on top implies it's a "long" vowel, even if you don't know IPA.

The "full" IPA form could be ɑe aɪ ɔɪ əi aː ai, or ɑ depending on whether you're British, American, Australian, Canadian, Irish, Welsh, Scottish, or whatever. You can see why Google would rather not get bogged down with all that.

Dictionaries usually include a full list of these simplified forms with examples somewhere at the front or back of the book, and often they list some of the most common ones at the bottom of each page for easy reference.

Certainly my British Collins and Chambers both use the same symbol as Google here, but I'm afraid I don't know a formal name for the entire symbol set. In fact, I rather doubt there is one, since obviously the whole reason IPA was created was to standardise things. That's when it got complicated because if you want to accurately show pronunciation, suddenly you need a whole bunch of additional symbols for something as simple as the vowel sound in sigh (sigh).