Early Buddhist Texts (such as the four principal Nikayas/Agamas) were in fact not established at once in complete structure (form) and content at the first Buddhist council.
The principal four Nikayas/Agamas were gradually developed and expanded from Samyutta-nikaya (SN)/Samyukta-agama (SA), according to Ven. YinShun. https://discourse.suttacentral.net/t/ve … hism/22540
The extant SA and SN, and also other Agamas/Nikayas, are sectarian texts. One can seek an understanding of early Buddhist teachings by studying them comparatively.
Nevertheless, the major early Buddhist teachings are shared in common in the extant SA and SN; e.g. see the following book by Choong Mun-keat:
The Fundamental Teachings of Early Buddhism: A Comparative Study Based on the Sūtrāṅga portion of the Pāli Saṃyutta-Nikāya and the Chinese Saṃyuktāgama (Series: Beitrage zur Indologie Band 32; Harrassowitz Verlag, Wiesbaden, 2000).
The particular collection of the Pali SN and the Chinese SA is mainly about knowing and seeing the four noble truths, the notion of anicca, dukkha, suñña (empty), anatta, and the middle way, which all are the core teachings of Early Buddhism.