How know what is the source needed for distribuite linux distros?

I know ubuntu have iso sources. But aside that, the big part of distro source is ditribuited by packpages. What i have to see for knowing i have all the source code of the distro needed for distribuiting it.

For example linux mint or ubuntu studio. I have not been able to find the source all glued together like ubuntu iso sources, it is all mixed with other packpages .