The plot could vary. Sometimes it's set in an alternate Hogwarts where the teachers come up with this baby project to promote unity among students. Draco and Harry, being two of the most prominent students from different backgrounds, are chosen. At first, they struggle with the idea and constantly bicker. However, through the challenges of taking care of the baby - like feeding it magic potions as food, putting it to sleep with spells - they find common ground and their relationship blossoms.
Well, in some Draco and Harry baby project fanfictions, it might involve the two characters being paired together for a school project that involves taking care of a 'baby' (which could be like a magic - created or a simulation). They start off disliking each other as they are from rival houses. But as they work on the project, they begin to understand each other better, and their relationship evolves from animosity to friendship or even something more romantic.