Question about Open-Source

This is a weird technical question that I have no clue about.

When a product is open source, how do you know that the program you are using is the open code that they published? Ex: Can a company release their code and say they are open source, but then just use different code for it? How can you check the code of a program?

Also, for programs that are closed source, why can't you just check the code of …

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