The coupling of electrons to bosonic excitations in solids, such as infrared-active phonons and carrier plasmons, has profound effects on the electronic properties of semiconductors and insulators. For example, bosonic coupling may result in the formation of polarons, i.e. electrons dressed by a boson cloud. Doped oxides constitute an exciting playground to investigate this phenomenon, as the polaronic nature of the charge carriers can strongly modify their optoelectronic properties. In angle-resolved photoemission spectroscopy (ARPES) the signature of polarons is the appearance of spectral satellites below the conduction band. Experimentally, these features evolve into band-structure kinks with increasing doping concentration, challenging our understanding of the many-body interactions in these systems [1, 2]. Here we present our approach to calculate ARPES spectra from first principles by combining ab initio calculations of the electron-phonon and electron-plasmon coupling with the cumulant expansion method [3, 4]. This allows us to investigate polaronic quasiparticles and their evolution with doping from first principles. For the paradigmatic example anatase TiO2, we show that the transition from polarons to a weakly-coupled Fermi liquid with increasing doping observed in experiments originates from nonadiabatic polar electron-phonon coupling. A similar mechanism also applies to the ferromagnetic semiconductor EuO . We observe that the coupling of electrons to low-energy carrier plasmons can sustain polaron formation analogously to the polar electron-phonon coupling. In particular, from combined ARPES experiments and ab initio many-body calculations we show that doped EuO can host plasmonic polarons with significant tunability with charge carrier doping.  S. Moser et al., Phys. Rev. Lett. 110, 196403 (2013).  Z. Wang et al., Nat. Mater. 15, 835 (2016).  C. Verdi, F. Caruso, and F. Giustino, Nat. Commun. 8, 15769 (2017).  F. Caruso, C. Verdi, S. Poncé, and F. Giustino, Phys. Rev. B 97, 165113 (2018).  J. M. Riley et al., Nat. Commun. 9, 2305 (2018).