Supermassive black holes at the centre of galactic nuclei mostly grow in mass through gas accretion over cosmic time .
This process also modifies the angular momentum ( or spin ) of black holes , both in magnitude and in orientation .
Despite being often neglected in galaxy formation simulations , spin plays a crucial role in modulating accretion power , driving jet feedback , and determining recoil velocity of coalescing black hole binaries .
We present a new accretion model for the moving-mesh code arepo that incorporates ( i ) mass accretion through a thin \alpha -disc , and ( ii ) spin evolution through the Bardeen-Petterson effect .
We use a diverse suite of idealised simulations to explore the physical connection between spin evolution and larger scale environment .
We find that black holes with mass \lesssim 10 ^ { 7 } M _ { \sun } experience quick alignment with the accretion disc .
This favours prolonged phases of spin-up , and the spin direction evolves according to the gas inflow on timescales as short as \lesssim 100 Myr , which might explain the observed jet direction distribution in Seyfert galaxies .
Heavier black holes ( \gtrsim 10 ^ { 8 } M _ { \sun } ) are instead more sensitive to the local gas kinematic .
Here we find a wider distribution in spin magnitudes : spin-ups are favoured if gas inflow maintains a preferential direction , and spin-downs occur for nearly isotropic infall , while the spin direction does not change much over short timescales \sim 100 Myr .
We therefore conclude that supermassive black holes with masses \gtrsim 5 \times 10 ^ { 8 } M _ { \sun } may be the ideal testbed to determine the main mode of black hole fuelling over cosmic time .