As a Senior Engineer (or Staff or Principal) coming into a new company, there will be some expectation that you are bringing in domain knowledge. Your opinion can be valuable. A few ways to have immediate impact:
As you ramp up, remember to "respect what came before." You don't want to disparage existing systems without realizing that it was best designed at the time of construction with the constraints and knowledge at the time.
As a senior engineer, you also need to lay the foundation for team and org-level impact. This means you invest extra time in building relationships and meeting other senior/staff/principal engineers.
Senior Engineer video from krazam: https://www.youtube.com/watch?v=eSqexFg74F8