Do you loose your mac book data When you upgrade the OS

No — however — if you do not back up your Mac, it means you have no concern about the prospect of permanently losing all the data on it.

That is unrelated to upgrading macOS, but the fact you asked the question suggests you may be concerned about your data.

Creating a backup is literally Step 1 in Update macOS on Mac - Apple Support.

Mine didn’t however it is always a good practice to backup your data first in the event that you do lose data and might require a clean installation of the new OS instead. You can opt for backups using Time Machine.

No, assuming your Mac is in proper working order.

No.