In the UK there have to be at least two examiners, one internal and one external. Both are important. It’s not true that the external has the final say or is senior to the internal. Both will have read your thesis. Both will be prepared. Together they will lead the discussion in the viva and together they will determine the outcome.
An independent chair could be part of the viva process: a member of staff who will observe and confirm that the viva was fair. Some universities always insist on a chair; others require them for certain situations. If there needs to be a chair then they will be there.
You might need your supervisor at your viva. It might feel appropriate to have them be a witness to a final step of your PhD journey. You might need them to make notes about what you discuss or you might need the help of having a supporter in the room.
Or you might need them to not be there: that’s fine too!
Finally, you need to be there. You need to be present, prepared and ready to engage.
Who is at the viva? In short, everyone who needs to be.