Google: Mobile-First Indexing Goes By Root Domain; Your Subdomains Move Over With Root

Google's John Mueller said on Twitter that when your root domain is moved over to mobile-first indexing, that means all your subdomains move with it. I suspect that this is the case for (a) sub-domains that are not like hosted WordPress sites that use subdomains for their URLs and (b) that the whole domain, including all the subdomains are ready for mobile-first indexing.