the integration function is in void src/core/integrate.cpp:214, integrate_vv(int n_steps, int reuse_forces),
the signle steps of the Velocity Verlet are actually in subroutines, which one does what can be seen from the comments.
The forces due to the Langevin thermostat can be found in src/core/thermostat.hpp:162 in inline void friction_thermo_langevin(Particle *p).
The relevant lines are starting at L292. By default we use flat noise and not Gaussian distributed. We use the classical RAN1 PRNG,
cf. Press, W.H. and S.A. Teukolsky, 1992; Portable Random Number Generators, implemented in src/core/random.hpp:81 for long integers, which
is converted to a double in the same file in the function d_random.