This is the APIs.io API search engine which searches across many other APIs.io search nodes, providing a federated API search engine that can be forked and run by anyone on cloud or on-premise GitHub.
APIs.json is a machine readable specification that API providers can use to describe their API operations, similar to how web sites can be described using sitemap.xml. APIs.json provides an index of internal, partner, and public APIs, which includes not just the the OpenAPI, JSON Schema, and other machine readable artifacts, but also the currently only human readable elements like documentation, pricing, and terms of service.
APIs.io uses APIs.json as the index for it's search engine, organizing many APIs.json by network nodes, helping break things up into manageable repositories. The central APIs.io network search node does not have any APIs.json, but will search across all of the search nodes, but each individual search node has all of the APIs.jon, OpenAPI, and other artifacts you are looking for as part of API searches.
APIs.io, APIs.json, and API Commons are projects led by Kin Lane and Steve Willmott, investing in the next generation of API discovery, providing not only an API search engine, but an open-source set of specification for enabling federated search. All of this work runs on GitHub, with the API Search organization providing all the links you need to get involved, contribute, and get any questions you might have answered.