Route 53, creating a Hosted Zone affect on existing URL traffic

There are two completely different things. One is registering a domain with Route 53 as the registrar, or transferring an existing domain from another registrar to Route 53. The second, separate thing is creating a hosted zone.

Domain registration or transfer from one registrar to another is a global operation and immediately affects the domain name across the world.

Creating a hosted zone has no effect on anything by itself. The zone will exist on Route 53's public DNS servers and be possible for anyone to access, but no one will look for the zone on those servers, until the domain registrar sets the parent domain (such as "com") to point your domain (like "example.com") to those new Route 53 DNS servers.

When you register a completely new domain (instead of transferring an existing one), it needs to be pointed to some DNS servers. That's why Route 53 will create an empty hosted zone by default and point the new domain there, unless you specify something different.

If you transfer the registrar role for your existing domain to Route 53 (instead of registering a new domain), the process will go as explained in this documentation article: https://docs.aws.amazon.com/Route53/latest/DeveloperGuide/domain-transfer-to-route-53.html.

The part relevant to your question is step 4 in the process: when taking over the registrar role for your domain, Route 53 will keep the DNS service pointed to the old DNS servers by default, and that's what you'll want it to do for the zone to remain pointed to your existing DNS servers.

But at step 4, you also have the option to replace the DNS servers with those of your new Route 53 hosted zone. It's generally better to make these changes separately, because it can be tricky or impossible to roll back changes while the old and new registrar are collaborating to hand over control over your domain name.