Does a spell that changes a permanent's type mid-resolution use the layer system or apply all at once?

When a spell or ability resolves, any continuous effects it generates (such as 'target creature becomes an artifact until end of turn') enter the layer system and are evaluated in the proper layer order alongside all other continuous effects. The layer system (CR 613) governs how multiple ongoing effects interact, not the resolution itself.

However, if a spell directly and permanently changes a permanent's type as part of its resolution — for example, a transforming effect with no duration — that change is simply applied at the moment of resolution as a one-time modification. It still enters the layer system's record if it creates a lasting continuous effect, but there's no 'conflict' mid-resolution since the spell resolves top to bottom sequentially (CR 608.2).

The layer system (CR 613.1) defines seven layers: copy effects (1), control-changing (2), text-changing (3), type-changing (4), color-changing (5), ability-adding/removing (6), and power/toughness (7). A type-change effect lives in Layer 4. When multiple effects apply in Layer 4, they are applied in timestamp order (CR 613.7a) or dependency order if applicable (CR 613.8).

Example: You cast Enslaved Horror's effect that makes a creature an artifact creature until end of turn, and another effect already makes it a land. Both effects are evaluated in Layer 4 together: the one with the earlier timestamp applies first, then the later one applies on top, so the final type is determined by their combined result in timestamp order — not by whichever spell 'finished resolving' first.

In short: the layer system is always the mechanism by which continuous type-changing effects coexist. A spell's resolution is simply the moment a new continuous effect enters that system; mid-resolution, instructions execute sequentially (CR 608.2c), but the resulting ongoing effects are always layered.