This study investigates the evolution of a Single-Stream Shear Layer (SSSL) originating from a wall boundary layer past a backward-facing step. Utilizing a time-resolved 3D-Particle Tracking Velocimetry (4D-PTV) technique, we track the trajectories of fluorescent particles to gain insight into the flow characteristics of the SSSL. A compact water-tunnel facility (Reτ = 1240) is fabricated to obtain an SSSL with a perpendicular slow entrainment stream past the separation edge. Spanwise-dominant coherent motion accompanied by finer flow scales is observed. The SSSL layer grows due to flow entrainment through "nibbling" actions of small-scale vortices, "engulfing" by large-scale vortices, and vortex-pairing events. Furthermore, the non-zero-speed stream edge grows considerably faster than the zero-speed stream edge, showing a strong asymmetry in mixing composition across a mixing layer. The SSSL reaches self-similarity around ≈ 60 θ0, where θ0 is the initial momentum thickness, a streamwise extent considerably shorter than suggested by previous studies. A turbulent kinetic energy (TKE) budget analysis reveals a negative production region immediately downstream of the separation edge attributed to a large positive streamwise gradient of streamwise velocity. In the self-similar region, the phase-averaged flow mapping demonstrates a larger concentration of turbulence production rate at the outer edges of spanwise vortices and the intersection of braids and vortices. Furthermore, a spatial separation exists in the regions of high production and dissipation rates within the vortex core region favoring dissipation. The braids exhibit a larger concentration of positive turbulence diffusion rates, indicating their function as a conduit for exchanging turbulence between neighboring coherent motions.