Australia

Parliament is able to legislate, like with literally everything in the constitution. Parliament could legislate that all pollies need to be holding a special duck blessed by the king's chief bum wiper to speak in the house if it wanted to.

Provided the constitutional requirements are fulfilled (there's a voice, they can make representations on indigenous issues) the precise nature of those representations, the composition of the voice etc are all a matter of legislation.

It's the same as every other constitutional requirement. The constitution provides minimum standards to meet and curtails certain powers, Parliament passes acts to fulfill those.

I doubt it. The whole "representation" part seems over-hyped. It's being promoted almost as if it will be a dedicated seat in parliament. The more likely outcome that it will just end up being a committee that reports into the existing NIAA structure and we don't end up seeing anything more impactful than what the NIAA is currently delivering.
If the Voice goes ahead, we can look forward to it running into the usual government bureaucracy, leading to disappointment once it becomes clear that government legislation doesn't solve issues that are occurring at the local, community level.