The development of 5G and Beyond 5G (B5G) technologies relies on the availability of experimentation facilities that can evaluate and validate the performance of these technologies. It is of great interest and challenge to design, deploy and operate large-scale experimentation platforms to meet the high requirements of various vertical use cases for the 5G services. This paper describes an i- CORA platform that we build with multiple partners in Norway to support several EU-funded projects (5GMediaHUB, IMAGINE-B5G, FIDAL and COMMECT) and vertical use cases. The platform is cloud-native and consists of four parts: a multi-vendor end-to-end 5G network with three RAN sites serving general use cases, two mobile private networks (MPNs) and three Networks on Wheels (NOWs) serving dedicated verticals, and an open source platform composed of open source solutions. i-CORA offers both advanced standalone 5G services and value-added services (e.g., security and testing) to verticals in Public Protection and Disaster Relief (PPDR), media, eHealth, Industry 4.0, etc. In this paper, we address the challenges and lessons learned during the implementation and operation of the i- CORA platform.