A typical assembly file is composed of links to the included components and assembly relationships. The components are parts or subassemblies that exist as separate files. When components are placed into an assembly file, links to those individual files are created in the assembly file. Initially a component is free-floating and can be moved and rotated in any direction unless it's grounded in place or constrained to other components.
For example, if you were to create an assembly of a simple drilled base plate and a bolt to be inserted in the hole, you would have three files: