## 1 Introduction  
Since the 1980s, digital 3D models of cultural heritage have been used in a variety of disciplines such as archaeology, architecture and art history, and museology as a tool for research and knowledge transfer. Such models are created primarily through either the retro-digitization of objects that still exist or the interpretative reconstruction of historical objects that no longer exist, based on multiple sources [1]. Although the working methods, data structures, and results of both approaches differ fundamentally [2], there are overarching requirements with regard to visualizing the 3D data contained and ensuring access to metadata and paradata sets.  
Within the European Union (EU), there has been a long-standing interest in 3D heritage. Related funding programs previously focused on technological aspects such as cost efficiency, user-friendly tools for creating digital 3D models [3] and their prototypic application, as well as the setup of infrastructures [4]. Overviews of projects and infrastructures in cultural heritage are provided by [5, 6, 7]. With regards to research infrastructures and their target groups, CLARIAH provides virtual research environments, especially for text-related research. ARIADNE and CARARE are dedicated to supporting archaeological information management [8, 9]. The European virtual library [10] is a repository for digital cultural heritage assets and collecting and aggregating institutions such as museums, libraries, and archives [11]. Complementing this, the E-RIHS infrastructure initiative focuses on tools and services for heritage sciences [12] (an extended overview is provided in [2]).  
Recently, 3D modeling of cultural heritage gained significant attention after the digital transformation of the cultural heritage sector after the COVID-19 pandemic [13] as well as the strengthening of the digital market through the Digital Europe Data Space [14, 15]. This is all linked to a shift toward large-scale digitization and modeling of 3D content. Consequently, the European Commission proposed a campaign in late 2021 to digitize all monuments and sites at risk as well as 50 percent of the most visited monuments, buildings, and sites in 3D by 2030 [16]. This would target 2.4 million 3D assets by 2025 and 16 million assets by 2030. Currently, in 2023, most accessible models are still stored in private sector repositories, such as Sketchfab, while public repositories contain comparatively few 3D model data to date (see Table 1).  
The purpose of this article is twofold. One aim is to analyze the current state and ecosystem for 3D models, particularly in Europe. This comprises (a) a review of the published studies on users, objects, and demands (b) an overview of an ecosystem for 3D heritage data. Both studies were conducted to derive implications for the design of the national infrastructure for the DFG 3D Viewer. As part of this distributed infrastructure project, the DFG 3D Viewer Jena experimental repository serves as a testbed for technology prototyping and testing. To overcome some of the challenges examined through the studies, we tested some prototypic approaches to (c) acquire 3D data from multiple sources, (d) enrich data quality, and (e) enable indexing, search, and viewing functionalities.