  We investigate the generation and evolution of switchbacks (SBs), the nature
of the sub-Alfv\'enic wind observed by Parker Solar Probe (PSP), and the
morphology of the Alfv\'enic transition, all of which are key issues in solar
wind research. First we highlight a special structure in the pristine solar
wind, termed a low Mach-number boundary layer (LMBL). An increased Alfv\'en
radius and suppressed SBs are observed within an LMBL. A probable source on the
Sun for an LMBL is the peripheral region inside a coronal hole with rapidly
diverging open fields. The sub-Alfv\'enic wind detected by PSP is an LMBL flow
by nature. The similar origin and similar properties of the sub-Alfv\'enic
intervals favor a wrinkled surface for the morphology of the Alfv\'enic
transition. We find that a larger deflection angle tends to be associated with
a higher Alfv\'en Mach number. The magnetic deflections have an origin well
below the Alfv\'en critical point, and deflection angles larger than
$90^{\circ}$ seem to occur only when $M_{\rm A} \gtrsim 2$. The velocity
enhancement in units of the local Alfv\'en speed generally increases with the
deflection angle, which is explained by a simple model. A nonlinearly evolved,
saturated state is revealed for SBs, where the local Alfv\'en speed is roughly
an upper bound for the velocity enhancement. In the context of these results,
the most promising theory on the origin of SBs is the model of expanding waves
and turbulence, and the patchy distribution of SBs is attributed to modulation
by reductions in the Alfv\'en Mach number. Finally, a picture on the generation
and evolution of SBs is created based on the results.
