I have several "categories" and so called "projects" (post of content type "project"). To each project corresponds a category.
What I call a "gallery", is several images, with each image attached several strings to (description, author).
To each category and to each project should be attached a gallery. If to a project a gallery isn't attached, on the project page the gallery of the category should be shown.
(Note that what I call here "categories" may or may not be the same as what is called "categories" in the manuals of WordPress.)
How to do it in WordPress?