Artificial planets would perform the same function as O’Neill space colonies and terraformed worlds: to provide a pleasant habitable environment suitable for people to live in on a permanent basis. Gravity is achieved by mass rather than centrifugal force. Location would be in a "Goldilocks zone" between the orbits of Mars and Venus, which broadly represents the region in which a planet will have a surface temperature compatible with a comfortable life.
The main material source for the planet is the Sun. The gas giants have two problems. A significant construction program would have a marked effect on the planet and the material is cold and would need to be directly mined. By contrast thousands of planets could be constructed from solar material without any significant impact on the sun and the solar material is in a plasma state which means it can be manipulated with magnetic fields. If a core build time is 30 years then 1019 kg of material would need to be delivered each day to the planet construction site. Proto-planet construction would have to take into account temperatures of materials, escape velocities, gravitational binding energies, energy inputs and radiating heat away from the planet, and time taken for the surface to cool.
The final construction phase of the planet would be terraforming by the introduction of a biological ecology by introducing suitable plants and animals and then giving them time to establish a working ecology. These could be done as isolated systems that would merge to form a global biosphere. A complete global coverage could be expected in less than 100 years although longer would be required for a mature biosphere.
Artificial planets require controlled fusion to produce heavy metals – mimicking stellar or supernova processes. The scale of the production facility must exceed the dimensions of the planet by at least an order of magnitude to accommodate the ingot accelerator and the radiator. Due to the heat generated by this construction process, it would take in the order of 10,000 years for the surface temperature to settle to a habitable value, the same as terraforming existing worlds.