internal

There you can see some internal stuff, that you won't usually use or have access to. You can modify PPLAF to make certain variables/functions accessible.

def.lua

This file contains some defines mostly for optimization purpose:

__DEF_FMATH_RANDOM_INT = fmath.random_int
__DEF_FMATH_RANDOM_FX  = fmath.random_fixedpoint
__DEF_FMATH_SQRT       = fmath.sqrt
__DEF_FMATH_ABS        = fmath.abs_fixedpoint
__DEF_FMATH_SINCOS     = fmath.sincos
__DEF_FMATH_ATAN2      = fmath.atan2
__DEF_FMATH_TO_INT     = fmath.to_int
__DEF_FMATH_TO_FX      = fmath.to_fixedpoint

You can see those definitions being used in PPLAF functions and some assets.