Radon enters through cracks in the slab, sump pits, gaps around plumbing/electrical penetrations, expansion joints, crawl space soil, and even through porous concrete itself. Indoor air pressure pulls it from soil gas.
More detail
Concrete is porous: a 4-inch slab passes a measurable amount of soil gas via diffusion alone. The dominant entry mechanism however is bulk-flow through paths created by construction or settling. The Cincinnati top-five entry paths in pre-1980 housing are: (1) the gap where the slab meets the foundation wall (expansion joint), (2) sump pit penetrations, (3) plumbing rough-ins where supply or drain lines come up through the slab, (4) cracks induced by foundation settling, and (5) for stone-basement homes, the mortar joints between stones in the basement walls. Modern slab-on-grade construction with sealed penetrations and a 6-mil polyethylene under-slab vapor barrier reduces but does not eliminate entry; even brand-new homes test above 4.0 pCi/L in Zone 1 territory at meaningful frequency. Cincinnati pre-1950 housing entry-path priority list: (1) the slab-to-foundation-wall expansion joint, (2) sump pit penetrations, (3) plumbing rough-ins (especially older floor drains), (4) settled cracks radiating from the center of the slab, (5) for stone-basement homes, mortar joints throughout the wall surface. The mitigator maps all five during the on-site assessment and addresses each via active depressurization plus targeted sealing.