You can customize the altitude of markers that you add to a 3D map.

The following code sample demonstrates how to set the altitude and altiude mode
of a Marker.
@State var extrudedMarker: Marker = .init(
position: .init(
latitude: 37.78980534,
longitude: -122.3969349,
altitude: 50.0),
altitudeMode: .relativeToGround,
collisionBehavior: .required,
extruded: true,
drawsWhenOccluded: true,
sizePreserved: true,
zIndex: 0,
label: "Extruded marker"
)