What materials are there for Srcum developer open or the PSPO is there none?
I notice a lot of materials on the PSM series but not the Developer and Product owner or Nexus. suggestions?
For Nexus, a good mastery of the PSM topics and a review of the Nexus Guide should cover it. Scaled Scrum is still Scrum, and it's best to think of it a a way to get around the 3-9 team member requirement. If you have a "team" of 18 people, you could split it up into three teams of 6 members, each with their own scrum process. Then the teams themselves are "nexus members" which also have their own scrum process. If you grasp that concept, then Nexus and Scrum become extremely similar.
PSD has some reading suggestions and public classes. In particular I found it important to focus on the Architecture, Test-first Development and Testing portions. While I can't recommend a specific book, those three concepts make lots of appearances throughout the exam., in addition to the basic Scrum framework.
Remember too that there's going to be a lot more material for the PSM series, because it is the direct competitor to the CSM/CSP certifications and the most popular CSM certification. More than 80% of Scrum.org certifications are issued in the PSM series, so they get the majority of the attention.