The Scrum Master in a Scrum Team is responsible for instilling quality by adhering to a Definition of Done.

When multiple Scrum teams are working on a product, they should select work from the same Product Backlog.

In a nutshell, Scrum requires a Scrum Master to foster an environment where:
The Scrum Team turns a selection of the work into an Increment of value during a Sprint.

Are changes that endanger the Sprint Goal achievement allowed?

In the Sprint Review, only the Developers, the Scrum Master and the Product Owner are allowed.