You can find various search modifiers in the help center. Or simply by clicking on "Advanced Search Tips" after you search for something.
Specifically, to find posts by a specific user you can user user:userid. And to find your own posts you can use user:mine or user:me.
Notice that if you go to some user's profile page, the search query user:userid is ...
From the FAQ post on migration: What is migration and how does it work? (in the section "Anything else I should know about migration?"):
Comments on the question that link to the homepage of the target site will be deleted. This is to remove redundant comments that tell the author to post on that site.
It is consistent with the linked post. In this case ...
If you follow the "Unanswered" link on the sidebar on the left of the page, then the page you get has options for sorting the unanswered questions. A direct link is: https://mathoverflow.net/unanswered/tagged/?tab=newest