The present study investigates streamwise (Farmula Presented) energy-transfer mechanisms in the inner and outer regions of turbulent boundary layers (TBLs). Particular focus is placed on the (Farmula Presented) production, its inter-component and wall-normal transport as well as dissipation, all of which become statistically significant in the outer region with increasing friction Reynolds number (Farmula Presented). These properties are analysed using published data sets of zero, weak and moderately strong adverse-pressure-gradient (APG) TBLs across a decade of, revealing similarity in energy-transfer pathways for all these TBLs. It is found that both the inner and outer peaks of (Farmula Presented) are always associated with local maxima in the (Farmula Presented) production and its inter-component transport, and the regions below/above each of these peaks are always dominated by wall-ward/away-from-wall transport of (Farmula Presented), thereby classifying the (Farmula Presented) profiles into four distinct regimes. This classification reveals existence of phenomenologically similar energy-transfer mechanisms in the 'inner' and 'outer' regions of moderately strong APG TBLs, which meet at an intermediate location coinciding with the minimum in (Farmula Presented) profiles. Conditional averaging suggests existence of similar phenomena even in low (Farmula Presented) canonical and/or weak APG TBLs, albeit with the outer-region mechanisms weaker than those in the inner region. This explains the absence of their (Farmula Presented) outer peak and the dominance of (Farmula Presented) wall-normal transport away from the wall, which potentially originates from the inner region. Given that the wall-ward/away-from-wall transport of (Farmula Presented) is governed by the Q4(sweeps)/Q2(ejections) quadrants of the Reynolds shear stress, it is argued that the emergence of the (Farmula Presented) outer peak corresponds with the statistical dominance of Q4 events in the outer region. Besides unravelling the dynamical significance of Q2 and Q4 events in the outer region of TBLs, the present analysis also proposes new phenomenological arguments for testing on canonical wall-turbulence data at very high (Farmula Presented).