I hit some shortcut and I can't find the setting the turn it off. But opening multiple files doesn't show different tabs.
Here's what I'm seeing
But this is what I'm expecting when I open a new tab
Check User interface / Tabs. You just need to add the following line in your settings.json file.
"workbench.editor.showTabs": false
I also think that How do I disable tabs in Visual Studio Code? is asking the exact opposite of what you are asking.