I don't understand when applying for job .. companies just post their product .. ask for improvements, bugs etc, :( Do I need to look over that I just skipped that types of jobs . Is this worth it to study their product and then apply ?

When they do there are two reasons.

First they might want to se if your reasoning is sound and that can be part of their evaluation.

Another reason is to take the opportunity to get free feedback, but Ido not think this is their primary reason since the whole process of trying to hire someone cost money to.

Like, share the code?

That seems like a bad idea...

Whatever, but if they pull the code from you for free, it's simply a scheme!