The B-21's shaping, materials, and construction are all leaps and bounds beyond the B-2, because the B-2 is fundamentally a late 1980s aircraft. All of that significantly contributes to RCS reductions. The B-21 is almost certainly quite a bit stealthier than the B-2.

Better understanding of material science lets you have better Radar Absorbent Material (RAM) performs significantly better than the old stuff while not constantly falling off and requiring massive climate controlled hangars. A perk of being able to piggy-back off development done for the F-35.

Better manufacturing lets you build construct shapes which previously would have been very difficult with greater precision. Panel gaps are not exactly great for stealth. The more manufacturing precision you have, the lower your RCS can be.

Better computer modeling lets you reduce RCS evenin both obvious and non-obvious ways. Highly detailed modeling of the full electromagnetic dynamics affecting the entire airframe are critical for achieving extremely high levels of broadband stealth and were totally impossible in the late 1980s but are doable now. In addition to designing a stealthy vehicle, that vehicle actually has to fly. You can see the differences in what's possible then and now in the engine intakes. Compared to the B-2 the B-21's engine intakes are incredibly deeply burred. They're mostly hidden even from straight on view and from even a few degrees below they're completely obscured from any observer. Making this work without starving the engines is very, very difficult and requires a lot of computational fluid dynamics. In the 1980s that shit very literally would not fly.