Do I need Nexus for PSMI or PPOI ?
As in topic ? Do I need to learn Nexus to get the scrum master or product owner test ?
You shouldn't require an in-depth knowledge of Nexus for the level 1 exams, as that has its own certification exam. However, understanding how Scrum works in multiple-team scenarios is very useful, and I would recommend looking over the Nexus Guide and the one diagram provided.
You will need to understand Nexus if you decide to sit for the level 2 or level 3 tests.
You may encounter some questions regarding multiple teams, but that does not equate to Nexus. It's important to bear in mind that "scaled Scrum is still Scrum," so the principles and values should still hold.