Nexus framework and integration team.

Hi Team i have few questions regarding nexus-

1)In nexus integration team there is one product owner as there is a single product backlog, wont the 3-9 smaller scrum teams consist of a product owner as a scrum team is made up of a scrum master,a product owner and dev team?

2)As mentioned in nexus guide,In nexus sprint planning appropriate representatives from scrum team and product owner meet to plan the sprint, who are the appropriate ppl from the scrum?Are they the senior developers?Or the scrum masters of the indiviual scrum teams?