## 1 Introduction  
In recent years, virtual reality (VR) and augmented reality (AR) have been established as revolutionary technologies capable of profoundly transforming a wide range of industries due to their innovative potential. They are now redefining the future of the academy and industry, becoming essential tools to improve education, production processes, design, and business operations. AR was introduced in Industry 4.0 as a key-enabling technology, and with the advent of the fifth industrial revolution (Industry 5.0), which aims to create a more human-centred work environment [1], promoting the collaboration between humans and machines [2, 3], technologies like VR and AR are taking on a more strategic role. In this context, VR and AR serve as crucial tools to improve human-machine interaction [4, 5], offering immersive experiences that support advanced training [6, 7], assisted maintenance [8, 9], real-time problem-solving [10, 11], and interactive modelling [12]. One particularly promising area for the application of VR and AR is their integration with Computer-Aided Design (CAD), where these immersive technologies enhance **how** designers and engineers create, interact with, and refine complex 3D models. The desire to merge 3D modelling tools, especially with AR [13], derives from the numerous advantages these tools offer [14], particularly when multimodal interfaces are available to interact with the software [15]. Direct interaction with the real world provides users with immediate spatial awareness of the dimensions of the model they are creating. Additionally, enabling multiple users to access a single 3D model simultaneously supports collaboration among designers, engineers, and other stakeholders, allowing them to view and interact with the project in real time, even in different locations. Project meetings and reviews become more efficient, as everyone can share a common vision of the model and suggest changes instantly. The ability to link real objects with the virtual model allows designers to see how products or structures integrate into the physical context. This helps to identify issues related to space, proportions, or interactions with the surrounding environment in a more intuitive way than simple on-screen visualisation, thereby reducing the likelihood of errors. The current state of the art presents several cases of implementing 3D modelling in VR/AR, each differing in terms of the type of mathematics used for solid modelling, the modelling method, or the framework used to provide input to the program. The use of a tilted interactive high-resolution display with multi-touch and pen input has enabled the development of immersive 3D modelling, allowing users to modify a primitive geometry through standard operations or model objects using extrusion or revolution from a 2D sketch [16]. Due to significant advances in consumer-grade AR and VR hardware and software, it is now possible to sketch 3D curves directly in mid-air. Several software tools have been developed based on generating surfaces through 3D sketches, using controllers [17; 18], motion-capture pens [19], hand tracking [20], or trackers and specialised VR gloves to monitor finger trajectories or hand movements [21]. Many of these conceptual design methods, such as sketch-based interfaces, require designers to construct all the surface edges before visualising the final surface as for a Boundary-Representation (B-Rep) approach. Other approaches use mesh sculpting [22], allowing users to model 3D objects from a rough geometry as if sculpting clay [23]. For these applications, techniques such as Octree [24], Voxel [25], and polygonal mesh [26] modelling have been used. In addition to these algorithms, other methods have been developed to facilitate sculpting, including those based on spring-mass systems using [27], mesh morphing [28], and Non-Uniform Rational B-Splines (NURBS) formulations [29]. Some of these applications have also included haptic devices to increase users' immersion in the virtual scene.