Im Confused About A Concept Of Opensource Projects?

Nobody pays them they do the projects as hobbies or in there spare time.

They volunteer their time and experience.
Why would someone do that? There are many reasons, but many of the best known projects (Wikipedia, Linux, gcc) the volunteers consider the end result to be more important than being paid. There is also the knowledge that their work will be valued by thousands or millions of users in the future.

Nobody. They’re written by volunteers.
Since O/S projects usually have no business model, they have no customer support and no “responsible” party. This is the primary reason large corporations avoid open source projects like Gimp and MySQL, and prefer using commercial products like Photoshop and Oracle even though they cost a lot of money.