How does DNS host like NameCheap secure and prevent anyone from creating an A record with your IP / domain name? [closed]

The only way I can think of is it surely checks the WHOIS record from the registrar to confirm the IP matches the domain name, but I can't find anything written about it.

Regarding DNS hosting services in general, the service provider requires you to authenticate as relevant administrative user (typically the user that created the zone, or possibly some other user designated by that zone creator) before you can manage your DNS zones, and you can only modify records in your own zones. That makes it pretty simple and clear who can do what.

As for IP addresses, there is no limit in DNS in general which domains can have address records (A/AAAA records) that point to which IP addresses, that is just not something that has any limit, but the consequences of this are generally not that big.

Now, regarding who gets to create a zone with a DNS hosting service in the first place, that can sometimes be a bit of a problematic thing.
Sometimes there is some form of validation of domain ownership, sometimes there isn't. Regardless, domains can change owners over time and unless the ownership check is somehow done on a regular basis it can be made invalid at any point.
That means there can be a situation where someone else may have created the zone first and effectively blocks the current domain owner from using that particular DNS service (without manual intervention).