  This thesis address theoretical and phenomenological aspects of active and
sterile mixing pattern within minimal extended seesaw frameworks. It consists
of six chapters, where chapters one and six are dedicated to introduction and
conclusion chapters, respectively. In chapter two, we study active-sterile
phenomenology with a single generation of sterile neutrino ($m_S\sim
\mathcal{O}$(eV)) along with the three active neutrinos. Three independent
cases for sterile mass matrices are studied in both normal and inverted
hierarchy mass ordering. Chapter three is an extension of the previous chapter.
The neutrino mass generation and baryogenesis {\it via} thermal leptogenesis
are studied in the fermionic sector. On the other hand, an extended multi Higgs
doublet model is studied in the scalar sector. Among the three Higgs doublet,
one of them does not acquire any vacuum expectation value (VEV); hence, it
behaves as an inert Higgs. The lightest component of this behaves as a viable
dark matter candidate. Within MES, sterile mass can be stressed up to the $keV$
scale. In the fourth chapter, we studied various phenomenologies considering
sterile neutrino mass in the $keV$ range. This $keV$ scaled sterile neutrino
also plays the role of dark matter candidate in our study. The fifth chapter is
dedicated to the texture-zero neutrino dark matter model. We study
active-sterile mixing, baryogenesis $via$ resonant leptogenesis and
$0\nu\beta\beta$ in the fermion sector. While, in the scalar sector, the
complex scalar flavon that gives rise to sterile mass takes part in dark matter
study. The imaginary component of that scalar flavon is behaving as a viable
dark matter candidate.
