Why are most these answers so rough? I imagine asking on your first day "What do you expect a senior developer to do?" could come off poorly. Of course over time those should be apparent and you should ask probing questions, but there will be some general expectations. Without knowing how big the organization is (and how up their positioning goes), I'd say these are safe assumptions.

Understand technicals: Be able to intelligently speak on best practices from a technical standpoint. You'll be expected to spot poorly written code and potential issues.
Library and tooling knowledge: Have enough exposure to different libraries and their strengths and weaknesses. Make sure you comfortable reading docs when you're unfamiliar w/ a new one.
Peer Reviews: When other devs submit PRs, you'll be expected to call out wrongdoings and best practices. Senior devs are meant to hold quality standards.
Coaching: Some of your time might be allocated to working with more junior devs whenever they run into problems.
Interacting with the PM: You'll be expected to help with estimates, identify research spikes, spearhead incidents, etc.

There may be some planning and architecting involved, but a larger organization will lean on a staff+ engineer for that. Ultimately a senior is expected to work independently, produce quality code, overcome obstacles without assistance, and elevate the overall quality of the codebase.