With the advent of 6G networks offering ultra-high bandwidth and ultra-low latency coupled with the enhancement of terminal device resolutions, holographic communication is gradually becoming a reality. Holographic digital twin (HDT) is considered one of the key applications of holographic communication, capable of creating virtual replicas for real-time mapping and prediction of physical entity states and performing three-dimensional reproduction of spatial information. In this context, integrated sensing and communication (ISAC) is expected to be a crucial pathway for providing data sources to HDT. This article proposes a four-layer architecture assisted by ISAC for HDT, integrating emerging paradigms and key technologies to achieve low-cost, high-precision environmental data collection for constructing HDT. Specifically, to enhance sensing resolution, we explore super-resolution techniques from the perspectives of parameter estimation and point cloud construction. Additionally, we focus on multi-point collaborative sensing for constructing HDT and provide a comprehensive review of four key techniques: node selection, multiband collaboration, cooperative beamforming, and data fusion. Finally, we high-light several interesting research directions to guide and inspire future work.