How do movies go about creating computer software or chat programs to use as props?

I worked for a local ABC affiliate that was owned by these horrible people. They did local commercials like the annoying used car ads that litter late night tv. A pretty common technique was to green screen the device and drop a overlay on it in post production. We used After Effects for this.

I just saw them do this a few weeks ago on Person of Interest. They used some random phone and put iOS on the screen. It happens a lot.