  We simulate the formation and collapse of prestellar cores at few-AU
resolution in a set of radiation-magneto-hydrodynamic simulations of giant
molecular clouds (GMCs) using the grid-based code RAMSES-RT. We adopt, for the
first time to our best knowledge, realistic initial/boundary conditions by
zooming-in onto individual massive prestellar cores within the GMC. We identify
two distinct modes of fragmentation: "quasi-spherical" and "filamentary". In
both modes the fragments eventually become embedded in a quasi-steady accretion
disk or toroid with radii ~ 500-5000 AU and opening angles $H/R \sim 0.5-1$.
The disks/toroids are Toomre stable but the accreted pre-existing fragments are
found orbiting the outer disk, appearing as disk fragmentation. Each core
converts nearly 100 percent of the gas mass into a few massive stars forming
near the disk center. Large and massive disks around high-mass stars are
supported by magnetic pressure in the outer disk, at radii >200-1000 AU, and
turbulent pressure in the inner disk. The most massive core accretes several
times more mass than its initial mass, forming a (proto)star cluster of 8
massive stars enshrouded by a toroid, suggesting a competitive accretion
scenario for ultra-high-mass star formation. We also find that the HII regions
produced by a single massive star remain trapped in the dense circumstellar
disks for a few hundred kiloyears, while the dynamic motions of massive stars
in wide binaries or multiple systems displace the stars from the densest parts
of the disk, allowing UV radiation to escape producing steady or pulsating
bipolar HII regions.
