If you're actually referring to a physical grain pattern on the image there are a few different ways this could happen. The Alexas sensor has a rather pleasing digital noise pattern, with certain exposures you end up seeing more noise but it looks kinda like film grain, which is cool.
Otherwise you can add grain in post production using a variety of products. Such as http://www.gorillagrain.com/ or http://rgrain.com/ However, it should be noted for future reference: If a camera such as a dslr is used for a shoot, and it has a bunch of blocky noise patterns, simply adding more grain on top of that won't make it better. You would need to "denoise" your footage first before adding a fake film grain. Something like https://www.neatvideo.com/ (my personal favorite, it's like magic) will get the job done.
Many productions add film grain in post because it gives it a certain look. Even Steve Yedlin ASC mentions in his resolution demos when comparing film to video that you can create a grain "algorithm" that matches film pretty much exactly.
We may never precisely know the methodology behind the way the film looks because workflows tend to be secretive, that way as a colorist or DI company you can offer something to your clients others cannot.