why are there combinations in katakana such as ティ to form “ti” or チェ for “che” but there aren’t any combinations for “tu”? also, why does japanese make these combinations when they aren’t used in hiragana? isn’t it a bit confusing for japanese people because these sounds are commonly used?
They exist! If you look in a Japanese-English dictionary for children, you’ll find entries like these:
do デゥ to テゥ
Do Japanese people find English confusing? Some do. Using katakana for pronunciation doesn’t make it any easier. But we all use the tools we have to make sense of the world.
"Tu" & "Du" have katakana combinations: トゥ nd ドゥ. They aren't very common, but they're used in words like cartoon (カートゥーン) and undo (アンドゥ). If you use a romaji keyboard, you can usually type them with "thu" and "dhu".
Some of the katakana combinations can't really be pronounced by monolingual Japanese people, but they're needed in order to differentiate foreign sounds from how they're normally approximated in Japanese. Even if they can't be pronounced, they're a visual cue that the word is actually foreign, as opposed to being a borrowed word into Japanese.
There's not exactly a "should be ..." for words written in Katakana, in the same way that many Spanish words in (American) English are pronounced "incorrectly" from a Spanish-speaker's perspective. When the words got their official reading, like tuna, it at that point became a Japanese word, and therefore, is "correct", if that makes sense.
