The u is pronounced, but it's a shorter sound than English u. Vowels in Japanese are shorter than English ones, which makes sense for a language where vowel length matters, eg su (vinegar) is a different word than suu (to smoke).

I'm trying to think of a word that wouldn't use u as an insert between 2 English consonants, like trip becomes torippu, but that's probably because they wanted to avoid tsurippu, as you mentioned about tsu. One word is garasu meaning glass (the material), but they also have gurasu, meaning drinking glass.

Most words do tend to use the u between English consonants though (supo-tsu, purezento, surippa, sute-ki, etc).

The kana alphabets were (probably) borrowed from Pali, and syllables follow the structure (consonant + vowel) or (vowel). In other words, a consonant must not, grammatically, occur alone. I don't know if Japanese still retains this as an explicit rule, but this is why you see the -u ending. It may or may not be pronounced, depending on which way flows better.

Many other languages with Pali / Sanskrit heritage have similar behaviour. However, Sanskrit itself and some modern languages have a dedicated character called a 'viraama', which says 'this consonant has no following vowel'. For example, in the word 'Padma' (lotus), the d is followed by a virama. Other languages, like Japanese, use 'u' instead of a dedicated viraama. So different languages in east, southeast and south Asia might write and say it as Padma, Paduma or Padama, but all versions would be mutually intelligible.