In your profile (see Help > About Opera for the location of your profile) there is a file named simply History (no extension). Though your history is not infinite - if you use Opera long enough some of the older data will be dropped. As far as format, it is some sort of database. I'm sure the date will be stored as an internet timestamp and hence not immediately readable, but you'd have to check with Chromium's file information to see the exact format.
What exactly do you mean by Chromium's file?
Is this a tool / program?
I took a screenshot. Is unfortunately in German.