New Relic One allows developers to build and deploy custom apps that provide unique views of data from various sources, catering to each company's specific business requirements. The platform showcases popular open-source apps, including Flex Manager, GraphiQL Notebook, and Attributory, which can be used out-of-the-box or customized with pull requests. To deploy these apps, developers require access to New Relic One, the CLI, a personal API key, Node v10 or higher, and a GitHub account. The process involves cloning or downloading the app from GitHub, configuring it, and deploying it through the CLI or by publishing an account-specific instance. New Relic provides various resources for building custom apps, including documentation, tutorials, and community forums, to help developers explore and contribute to open-source projects.