The answer is Pope Julius II's Life.
They could have used things like trees, people, animals, mountains, etc as a reference/starting point for their work.
Literature, painting, sculptures & music
Minimum - a tendency that some have referred to as the "dematerialization" of art.Conceptual artists were influenced by the brutal simplicity of Minimalism, but they rejected Minimalism's embrace of the conventions of sculpture and painting as mainstays of artistic production. For Conceptual artists, art need not look like a traditional work of art, or even take any physical form at all.The analysis of art that was pursued by many Conceptual artists encouraged them to believe that if the artist began the artwork, the museum or gallery and the audience in some way completed it. This category of Conceptual art is known as 'institutional critique,' which can be understood as part of an even greater shift away from emphasizing the object-based work of art to pointedly expressing cultural values of society at large.Much Conceptual art is self-conscious or self-referential. Like Duchamp and other modernists, they created art that is about art, and pushed its limits by using minimal materials and even text.