The runtime session outlives closed tabs. Even if you sign out from your google account and minutes later come back and login again, your notebooks still stays at the same point, since the VM holding the kernel still runs.
Two years ago someone here on stackoverflow said that it would remain for 90 minutes if you close your tab and 12 hours if you don't - Google Colab session timeout.
I don't know if still holds. At least on their FAQ, google does not guarantee anything. In https://research.google.com/colaboratory/faq.html they only say "Virtual machines are deleted when idle for a while, and have a maximum lifetime enforced by the Colab service."
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