Accretion and jets from stellar-mass to supermassive black holes

Accretion and jets occur in many astrophysical systems across a multitude of size and mass scales, and environments. As such, the study of accretion and jet physics has for decades been, and still remains, a hot topic in astrophysics. Accretion onto black holes has particular significance for many reasons, not least because supermassive black holes likely exist at the centres of every galaxy in the universe. The energetic impact of black hole accretion is therefore key to furthering our understanding of the universe as a whole. Over the past few decades we have learned that black hole accretion seems to be a scale-invariant process: despite the orders of magnitude difference in black hole mass, and the diversity of astrophysical environments accretion occurs within, the efficiency and power budget associated with the accretion process appears ignorant to these variables. However, the physical explanation for this scaling is lacking, due to the degeneracy in broadband spectral modelling. In this thesis I show work I have done, alongside many collaborators, to address these modelling degeneracies. I focus on the outflow-dominated states of accreting black holes, from the stellar-mass components of X-ray binaries in the Milky Way, to their supermassive analogues in low-luminosity active galactic nuclei. In addition, I show how this concept of scale-invariance can be used to make predictions about the existence of the more controversial primordial black holes.