Instances that disagree with being found in search engines are not shown. Instance admins can configure their robots.txt by adding lemmy-search. All other instances can theoretically be found. I think their priority depends on the laws of SEO (Search Engine Optimization). This probably means that a post on myownlemmy1337 that is federated with lemmy.world, will be found as a post on lemmy.world.

So, if Lemmy was very famouse, I guess it’s possible to get pages over pages with the same result from different instances. However search engines usually have a way to exclude “similar” results.

For voyager it may be possible, they do not want to be found, I don’t know about this though. You could add site:vger.app to your search prompt for testing this.