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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 ...
4
It seems that you simply switched to mobile web by mistake. It is possible to switch between mobile version and full site, currently you can find the switch near the bottom of the page.
In the mobile version, you should click on "full site" to change the interface. When on full site, you can get to mobile version by clicking on "mobile".
Here is a ...
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Here is how I do it.
I click on my own little user icon at the top of the page
and this puts my own userid into the search field
where I can then add keywords I want to search for.
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