What do my employers check in a pre employment background check for entry level jobs? I provided only education details in the background check portal.?

I recently received a conditional job offer for an entry level position. And as part of the process, I submitted a background check application where I provided only my sin number and education records. They didn't ask me for any employment records. Does this mean they will check only for criminal and educational background? The other concern I have is with my previous employer, where I worked voluntarily for 9 months to gain experience even after the official contract ended. In my resume, I included that 9 months as work experience because it is. But during the BG I am pretty sure there will be no record of my payable employment there for 9 months. But I worked there and I can get letter of reference from my professor confirming it. How do I tackle this situation with my future employer if it comes up? I'm not trying to cheat. I worked in that lab, but there is no proof other than my professor's official letter.

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