Phonology is Study of speech sounds isolation,phonology is more focused on how  speech sounds change and how behave when in a syllable, word or sentence.

Try saying the word 'helps' out loud, paying close attention to the final sound of the word. After that, say the word 'crabs' out loud, again paying attention to the final sound. After getting over the embarrassment of talking to yourself (especially if you're in a crowded place!), what did you notice about the final sounds of these words?

If you said that they are different, you're absolutely correct! In 'helps', the final sound is pronounced like you would expect the letter 's' to sound. However, in 'crabs', the ending should have sounded more like a 'z'. The reasoning for this change can be found through the use of phonology, which is the study of speech sounds and how they change depending on certain situations or placements in syllables, words, and sentences.