How to use Junction Object in project?

Hello Manpreet,

Junction object is an object which helps to create many to many relationship. It can be achieved through 2 master-details relationships with 2 masters and a common detail(child) object between them.
The comon child is called as Junction object.

Is Parent Project, Child Project & Project are different objects? If not, then a simple lookup will serve the purpose. If you want to connect a Project with parent and child projects, create lookups on Project.

Also you can create a new custom object to store child Project. You can have master- child relationship where your main Project will be act like parent and child project will act like child project. As it's one to many, one Project can have multiple child project.