  During the ultraintense laser interaction with solids (overdense plasmas),
the competition between two possible quantum electrodynamics (QED) mechanisms
responsible for $e^\pm$ pair production, i.e., linear and nonlinear
Breit-Wheeler (BW) processes, remains to be studied. Here, we have implemented
the linear BW process via a Monte Carlo algorithm into the QED particle-in-cell
(PIC) code YUNIC, enabling us to self-consistently investigate both pair
production mechanisms in the plasma environment. By a series of 2D QED-PIC
simulations, the transition from the linear to the nonlinear BW process is
observed with the increase of laser intensities in the typical configuration of
a linearly polarized laser interaction with solid targets. A critical
normalized laser amplitude about $a_0\sim$ 400-500 is found under a large range
of preplasma scale lengths, below which the linear BW process dominates over
the nonlinear BW process. This work provides a practicable technique to model
linear QED processes via integrated QED-PIC simulations. Moreover, it calls for
more attention to be paid to linear BW pair production in near future
10-PW-class laser-solid interactions.
