Phonemes are 'set', a bit like graphemes (written sounds) - CA in catcher is separated into K and A because C isn't always pronounced CA. The same as SH in ship would be a separate phoneme to the I sound, because SH isn't always pronounced SHI.
I don't know if this is helping 😆
The 44 phonemes we use in English are
Consonant Phonemes: /b/, /d/, /f/, /g/, /h/, /j/, /k/, /l/, /m/, /n/, /p/, /r/, /s/, /t/, /v/, /w/, /y/, /z/, /ch/, /sh/, /th/ (voiced), /th/ (unvoiced), /zh/, /ng/.
Vowel Phonemes: /a/, /e/, /i/, /o/, /u/, /ae/, /ee/, /ie/, /oe/, /ue/, /oo/, /oo/, /ou/, /oi/, /ar/, /er/, /or/, /air/, /eer/, /are/.