Are desktop PC programs created differently than mobile programs?

I've heard about a growing "shift to mobile." In terms of programming/design, is it THAT different? Why would a program like Facebook, for example, be different on a mobile device like a phone vs. its use on a PC. I don't get the big fuss.
Update: Like, do you have to write the program in a totally different language or something to have it work on mobile? I'm tech illiterate, so if you can explain in layman's terms that'd be great. I ask this b/c I am learning about stocks and it's a common thing I see in news of tech companies.
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