Emergence of high-mass stars in complex fiber networks (EMERGE). I. Early ALMA Survey: observations and massive data reduction

(Abridged) Recent molecular surveys have revealed a rich gas organization of sonic-like fibers in all kind of environments prior to the formation of low- and high-mass stars. This paper introduces the EMERGE project aiming to investigate whether complex fiber arrangements could explain the origin of high-mass stars and clusters. We analyzed the EMERGE Early ALMA Survey including 7 star-forming regions in Orion (OMC-1/2/3/4 South, L1641N, NGC2023, and Flame Nebula) homogeneously surveyed in both molecular lines (N$_2$H$^+$ J=1-0, HNC J=1-0, plus HC3N J=10-9) and 3mm-continuum using a combination of interferometric ALMA mosaics and IRAM-30m single-dish (SD) maps. Based on our low-resolution (SD) observations, we describe the global properties of our sample covering a wide range of physical conditions including low-, intermediate, and high-mass star-forming regions in different evolutionary stages. Their comparison with ancillary YSO catalogs denotes N$_2$H$^+$ as the best proxy for the dense, star-forming gas in our targets showing a constant star formation efficiency and a fast time evolution of <1 Myr. While apparently clumpy and filamentary in our SD data, all targets show a much more complex fibrous substructure at the enhanced resolution of our ALMA+IRAM-30m maps. A large number of filamentary features at sub-parsec scales are clearly recognized in the high-density gas traced by N$_2$H$^+$ directly connected to the formation of individual protostars. This complex gas organization appears to extend further into the more diffuse gas traced by HNC. This paper presents the EMERGE Early ALMA survey including a first data release of continuum maps and spectral products for this project to be analysed in future papers of this series. A first look at these results illustrates the need of advanced data combination techniques to investigate the intrinsic multi-scale, gas structure of the ISM.