Check DNS propagation

As long as the previous record's TTL says, and no longer. If the TTL was 3600 seconds, within an hour the last resolver holding it cached will have dropped it. The “24 to 48 hours” figure comes from an era of very long TTLs and batched zone transfers; today it is mostly folklore.

Because each one cached its copy at a different moment and each countdown runs independently. A resolver that asked just before the change keeps the old value until it expires; one that asked afterwards already has the new one. There is no wave sweeping across the planet: just caches expiring on their own schedules.

It helps your computer and nothing else. It drops the local copy, but if your ISP's resolver still holds the old value it will hand it right back on the next query. That is why this tool asks public resolvers: to show what the rest of the world sees, not what your laptop sees.

That nothing is published for that name and that type. It is normal if you have just created it and it has not left the zone yet, if you got the subdomain wrong, or if you are simply asking for a type that domain does not use: plenty of domains have no AAAA, and a subdomain with an A record has no CNAME.

No. The query goes to the resolvers, the answer comes back to you, and that is the end of it. Results are held in memory for a few seconds so that hitting refresh does not fire another twenty queries, and they expire on their own. There is no database, no log of queried domains, and no cookies for any of this.