U.S. Tax Filing as a Canadian Resident Working in the U.S.

I am a Canadian citizen living in Canada, and I cross the border to work in the United States. This is my first time filing U.S. taxes, and I have the following questions:

When asked about household filing status, should I choose “plan to file separately,” since my wife lives with me in Canada?

In the personal information section, when asked about my spouse and dependents’ SSN, I do not have SSNs for them because I am the only one working in the U.S. Should I leave those fields blank?

Where do I report my 401(k) contributions?

If I am eligible for a refund, how will I receive it?

Thank you for your assistance.