We report a combined theoretical and experimental investigation devoted to get deeper insights on the exotic magnetic properties of the low-dimensional SeCuO3 system, for which the two inequivalent Cu(1) and Cu(2) sites show different quantum dynamics. First-principles calculations based on the density functional theory were performed to extract the magnetic exchange couplings. Briefly, we notably evidenced that i) the magnetic structure can be decomposed into two subsystems made by strongly antiferromagnetically (AFM) coupled Cu(1) singlet state dimers and weak AFM Cu(2) spin chains, and ii) weak ferromagnetic (FM) interactions between the two subsystems lead to magnetic frustration. The present model allows to reproduce both magnetic susceptibility and torque magnetometry measurements. In addition, high-magnetic field experiments and DMRG simulations evidenced a half-magnetization plateau at 40-45 T associated to the polarization of the Cu(2) spin-chains, while the Cu(1) dimers are expected to reach the triplet state at 210-220 T.