what is Orion? N-Central?

Orion is not a product. It's a platform shared by several active SolarWinds products. See this page for a list of some of them: IT Monitoring Software | SolarWinds Orion Platform

That Super User question has a link to a youtube video showing how to get started and run some queries.

I'm looking at:

he Orion Platform is at the core of the SolarWinds IT Management Portfolio. It provides a stable and scalable architecture that includes data collection, processing, storage, and presentation. The Orion Platform provides common features like network node discovery, dashboards, reporting, alerting, SNMP traps, Syslog, groups, and more that can be leveraged across all products.

Learn more about the benefits of unified IT monitoring with the SolarWinds Orion Platform​

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which leads me to infer that, if Orion is at the core of this portfolio, that it's API is within N-Central.  Certainly N-Central has an API:

https://support.solarwindsmsp.com/kb/solarwinds_n-central/Accessing-SolarWinds-N-central-API-functions

I was just looking for direct documentation on this API and trying to understand the relationship, if any at all, with Orion.

At least some of the documentation for N-Central API's seems to require a login, whereas Orion is on github.

Your inference is incorrect, Ncentral is a product built from an acquisition of a client hosted remote management platform, formerly known as N-Able.  It stands entirely on it's own and has never integrated with Orion in a meaningful way.  Likewise they bought GFI Logicnow, which is was integrated with Ncentral.  They are actually a subsidiary company called SolarwindsMSP and are pretty well separated from the Orion suite.

Orion is the platform that has, for the most part, been the result of internal development as a client hosted set of infrastructure monitoring tools.

Solarwinds as a company has their toes in many different pools of clients, something like 40 product lines and the ones that were picked up via acquisitions of other companies products tend to be their own thing for some time after purchase until they can figure out if there is a clean way to integrate them into Orion (STM, VMAN, and DPA used to be totally separate, and over time became embedded in Orion).  Some products remain separate though, such as the Kiwi tools.

https://en.wikipedia.org/wiki/SolarWinds#Acquisitions​