Google's Gary Illyes said that hreflang alternates, hreflang URLs are not "indexed in the proper sense." In short, Google just keeps the hreflang URL as alternate URLs. So the URL is not what is getting the page indexed, it is just being stores as an alternative to the main page URL.
Gary said this in response to a post on LinkedIn where Faiez Javaid asked:
If Google is choosing by itself one /lang/ as canonical and ignoring others showing in GSC as not indexed, then how can Google show other lang versions of the same page to the user in Search results?
Gary Illyes full answer was:
There's two things at play:1. alternate names: when we canonicalize a URL, the other URLs in its dups cluster it was chosen from may become so called alternate names. They may be used in search results if the user query deserves one of those alternate names. The most prominent use case is the "site:" queries. If you do a search like [site:bit.ly], we may show a bunch of urls that actually redirect; they're not indexed in the proper sense, they're just alternate names of a canonical url.
2. hreflang alternates become such alternate URLs: they're not in fact indexed in the proper sense.
While the specific URL might not be indexed, the URL is mapped to the canonical page that is indexed.
Forum discussion at LinkedIn.
