How does the Litecoin difficulty get calculated?
Litecoin is a fork of Bitcoin Core, and uses the same difficulty adjustment period (2016 blocks), which on Litecoin is 3.5 days because the block time is 2.5 minutes instead of 10.  See chainparams.cppL#85:

consensus.nPowTargetTimespan = 3.5 * 24 * 60 * 60; // 3.5 days
consensus.nPowTargetSpacing = 2.5 * 60;