Has my DNS propagated?

As long as the TTL of the previous record, not “up to 48 hours”. If the old record had a TTL of 3,600 seconds, any resolver that looked it up ten minutes ago will keep serving it for another fifty. The 48 hours were the worst case twenty years ago, when one-day TTLs were the norm; today most changes are visible within minutes.

Because each keeps its own copy. The one that had not asked about your domain recently goes to the source and gets the new value; the one that asked a moment ago keeps serving the old one until it expires. There is no central process that “pushes” the change: propagation is really expiry.

On other people’s resolvers, no. Cloudflare and Google offer forms to purge the cache for a specific name, which helps anyone using those resolvers. What you can control is prevention: lower the TTL to 300 seconds a day or two before the change, make the change, and raise it again once everything is stable.

The time to live, in seconds, that a resolver may keep an answer before asking again. A high TTL saves lookups and makes the site faster to resolve; a low one makes changes visible sooner. Here you see the remaining TTL each resolver reports, which counts down to zero.

There are more caches along the way: your ISP’s or router’s resolver, your operating system’s DNS cache and the browser’s own. Switching your device’s DNS to 1.1.1.1 or 8.8.8.8, or opening the site from a phone on mobile data, usually confirms the change is done and only your copy has yet to expire.

An A record points straight at an IP address; a CNAME points at another name, which is then resolved in turn. A CNAME is convenient when a provider changes IPs without notice, but it cannot coexist with other records on the same name, which is why the domain root rarely uses one. When you look up a CNAME, the resolver also returns the A records of its target.

That the name does not exist for that resolver. After creating a new subdomain it is normal to see it on one of them for a while: negative answers are cached too, with the TTL set by the zone’s SOA record. If you see it on all three right after creating the record, check that you created it in the right zone.

Not here: the lookups go straight from your browser to the three public resolvers, just as when you open any website. Each applies its own privacy policy; Cloudflare and Quad9 commit to not storing the IP of whoever asks.