Scrum.org provides a Learning Path for Scrum Masters. It links to videos, blogs, books and other helpful things to help you prepare.
The PSM assessment is pretty strict on the Scrum Guide's guidance on working with multiple teams:
The Product Owner is the sole person responsible for managing the Product Backlog.
Multiple Scrum Teams often work together on the same product. One Product Backlog is used to describe the upcoming work on the product. A Product Backlog attribute that groups items may then be employed.
If "Done" for an increment is not a convention of the development organization, the Development Team of the Scrum Team must define a definition of "Done" appropriate for the product. If there are multiple Scrum Teams working on the system or product release, the Development Teams on all the Scrum Teams must mutually define the definition of "Done".
A Sprint Review is held at the end of the Sprint to inspect the Increment and adapt the Product Backlog if needed.
When applying Scrum to scale Scrum, these basic rules officially still apply. Nexus and LeSS stay mostly within this set of constraints. Nexus+ and LeSS Huge break some of these constraints. SAFe by default overrules all of these constraints.