Do I need to keep content away from the fold?

Yes, when the device is partially folded. The hinge divides the inner display into separate usable regions, and content across that division is physically bent away from the viewer.

The API for this is ReservedRegion in SwiftUI and UIViewReservedRegion in UIKit, querying kind: .division. The important behaviour is that a division region is active only while the device is folded and has zero width when flat, so your layout must handle regions appearing and disappearing rather than simply moving.

For most main-and-secondary layouts you do not need to query regions directly. ArrangementView already takes active division regions as an input and places your views accordingly.

Division versus occlusion

Reserved regions come in two kinds, and the distinction decides how you respond:

• Division — splits an area in two. The hinge. Active only while folded, zero width when flat.
• Occlusion — covers an area. The under-display FaceTime camera. Active only while the camera is active, inactive otherwise.

GeometryReader { proxy in
    let fold   = proxy.reservedRegions(kind: .division)
    let camera = proxy.reservedRegions(kind: .occlusion)
}

For a main-and-secondary screen you usually do not need to query at all — ArrangementView already takes active division regions as an input.