The study of various lepton number violating (LNV) and lepton flavour violating (LFV) processes is one of the most important gateways to probe and understand the Dirac vs Majorana nature of neutrinos and various mass models of neutrino mass generation. Neutrinoless double beta decay is one of the most important modes beyond the SM to probe the Majorana nature of neutrinos. I will discuss how the experimental searches for neutrinoless double beta decay with emission of a light exotic scalar neutral particle (with a signature of anomalous missing energy) is sensitive to an effective dimension-7 operator with a (V + A) lepton current and (V ± A) quark currents, which can naturally arise in many popular Beyond the Standard Model (BSM) scenarios such as Left-Right Symmetric model, R-parity violating supersymmetry and leptoquarks. I will also discuss how various meson and tau factories can play a crucial role in probing and constraining neutrino mass models. Several BSM scenarios often introduce additional sterile (w.r.t. the SM gauge group) neutrinos to explain the neutrino masses. Using the effective sterile neutrino framework, I will discuss how in the presence of more than one Majorana fermion an interference effect of LNV and LFV semileptonic meson decays can lead to interesting implications for current experimental constraints and possible future observations. In particular, how the CP violating phases can lead to constructive and destructive modulation of lepton number violating and conserving modes, necessitating a more careful reinterpretation of the current exclusion limits in the presence of more than one generation of Majorana neutrino.