Mrk 421 is prominent by the extreme X-ray and TeV-band variability on diverse timescales ranging from years down to a few minutes. Our intense X-ray spectral and timing study revealed the features of different physical processes: enhanced collimation rate of high-energy plasma through the relativistic jet in some epochs; extreme electron energy distribution (EED) with the high-energy cut-off extending to the Lorentz factors of 10^6; turbulence-driven magnetic reconnection in relativistically magnetized jet area with very small spatial extents of ~10^13 cm; transition from the Kraichnan-type into the 'hard-sphere' turbulence; injection of high-energy electron population with a hard energy distribution; possible presence of hadronic cascades and random fluctuations in the particle acceleration rate; interaction between the relativistic shock front and jet inhomogeneities with strong magnetic fields. Some other experimental results are explained by first-order Fermi acceleration with very low initial particle energy distribution and/or shock propagation in the magnetized jet area with different confinement efficiency.