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Using the final MaNGA sample of 10K galaxies, we investigate the dark matterfraction $f_{\rm DM}$ within one half-light radius $R_{\rm e}$ for about 6Kgalaxies with good kinematics spanning a wide range of morphologies and stellarvelocity dispersion. We employ two techniques to estimate $f_{\rm DM}$: (i)Jeans Anisotropic Modelling (JAM), which performs dark matter decompositionbased on stellar kinematics and (ii) comparing the total dynamicalmass-to-light ratios $(M/L)_{\rm JAM}$ and $(M_{\ast}/L)_{\rm SPS}$ fromStellar Population Synthesis (SPS). We find that both methods consistently showa significant trend of increasing $f_{\rm DM}$ with decreasing $\sigma_{\rm e}$and low $f_{\rm DM}$ at larger $\sigma_{\rm e}$. For 235 early-type galaxieswith the best models, we explore the variation of stellar initial mass function(IMF) by comparing the stellar mass-to-light ratios from JAM and SPS. Weconfirm that the stellar mass excess factor $\alpha_{\rm IMF}$ increases with$\sigma_{\rm e}$, consistent with previous studies that reported a transitionfrom Chabrier-like to Salpeter IMF among galaxies. We show that the$\alpha_{\rm IMF}$ trend cannot be driven by $M_{\ast}/L$ or IMF gradients asit persists when allowing for radial gradients in our model. We find noevidence for the total $M/L$ increasing toward the centre. We detect weakpositive correlations between $\alpha_{\rm IMF}$ and age, but no correlationswith metallicity. We stack galaxy spectra according to their $\alpha_{\rm IMF}$to search for differences in IMF-sensitive spectral features (e.g. the $\rmNa_{\rm I}$ doublet). We only find marginal evidence for such differences,which casts doubt on the validity of one or both methods to measure the IMF.

MaNGA DynPop -- V. The dark-matter fraction versus stellar velocity  dispersion relation and stellar initial mass function variations in galaxies:  dynamical models and full spectrum fitting of integral-field spectroscopy