Billboarding System/Component for RealityKit (you supply headPosition)

BillboardSystem.swift

import RealityKit

struct BillboardSystem: System {
    
    static let query = EntityQuery(where: .has(BillboardComponent.self))
    
    init(scene: RealityKit.Scene) {}
    
    func update(context: SceneUpdateContext) {
        context.scene.performQuery(Self.query).forEach { entity in
            
            guard let billboard = entity.components[BillboardComponent.self] else { return }

            let headPosition = ARData.shared.headEntity.transform.translation
            let entityPosition = entity.position(relativeTo: nil)
            let target = entityPosition - (headPosition - entityPosition)
            
            if let upVector = billboard.upVector {
                entity.look(at: target, from: entityPosition, upVector: upVector, relativeTo: nil)
            } else {
                entity.look(at: target, from: entityPosition, relativeTo: nil)
            }
        }
    }
}
    
struct BillboardComponent: Component {
    var upVector: Float3?
    
    init(upVector: Float3? = nil) {
        self.upVector = upVector
    }
}