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We use the GALFORM semi-analytical model of galaxy formation and thePlanck-Millennium simulation to investigate the origins of stellar mass ingalaxies and their spheroids. We compare the importance of mergers and discinstabilities, as well as the starbursts that they trigger. We find that thefraction of galaxy stellar mass formed \textit{ex situ} ($f_\mathrm{ex}$)increases sharply from $M_*=10^{11}$ M$_\odot$ upwards, reaching $80\%$ at$M_*=10^{11.3}$ M$_\odot$. For low-mass galaxies we find larger\textit{\textit{ex situ}} contributions at $z=0$ than in other models($7$-$12\%$), with a decrease towards higher redshifts. The global \textit{exsitu} fraction of all stellar mass falls sharply with redshift, from $40\%$ at$z=0$ to $3\%$ at $z=10$. Major mergers contribute roughly half of the\textit{ex situ} mass, with minor mergers and smooth accretion of satellitesboth accounting for $\approx25\%$, almost independent of stellar mass andredshift. Mergers dominate in building up high-mass ($M_\mathrm{*,sph}>10^{11}$M$_\odot$) and low-mass ($M_\mathrm{*,sph}<10^{8.5}$ M$_\odot$) spheroids. Discinstabilities and their associated starbursts dominate for intermediate-massspheroids ($10^{8.5}<M_\mathrm{*,sph}<10^{11}$ M$_\odot$) at $z=0$. The massregime where pseudobulges dominate is in agreement with observed pseudobulgefractions, but the peak value in the pseudobulge fraction predicted by GALFORMis likely too high. The total contributions of disc instabilities and theirstarbursts are roughly equal at $z=0$, with the former dominating forlower-mass spheroids (peak at $M_\mathrm{*,sph}=10^{9.5}$ M$_\odot$) and thelatter for higher-mass ones (peak at $M_\mathrm{*,sph}=10^{10.5}$ M$_\odot$).

The buildup of galaxies and their spheroids: the contributions of mergers, disc instabilities and star formation