When creating syllables we basically require an onset, nucleus, and a coda. Now, usually, all language have an onset, and the coda is fairly optional. Before creating a phonetic system, we first decide the word order, and rules about what sounds are allowed in the onset, nucleus and coda positions.
My question is focused on the 2nd part. For example, in English, the sound [ŋ] isn’t allowed to begin a word, but it can begin a syllable; and in Dothraki, the consonants [ɡ], [q], and [w] can’t end a word.
So these restrictions, do they arrive because of some IPA rule? Or is it the developer that creates these rules?