3D printers build models in layers, which you can see if you look at a model closely. If you want a smoother, shinier surface, you can achieve injection mould-like quality with a little acetone.
Acetone is a powerful solvent that can actually break down acrylonitrile butadiene styrene (ABS), which is a plastic commonly used in 3D printing. And as Matthew Perks demonstrates in this video from his YouTube channel, you can use the vapors acetone gives off to smooth a model's hard edges. To do it, you'll need a tray, a tall glass, some acetone and a small metal platform for your model to sit on. You'll also need a small piece of cardboard and a paper towel.
Set the model and platform on your tray, then fold a paper towel into a loop and put it at the bottom of the glass. Wedge it in place with the cardboard piece, and pour a little acetone in the glass so the paper towel can absorb it. Now place the glass over the model and fill the bottom of the tray with a little water so the acetone vapours don't seep out. After some time passes and your model starts to look shiny, just remove the glass and let the model dry.