What is .xyz in relation to a url my Apple TV visited?

That .xyz is a top-level domain, same as .com, .net, .org, and a ~thousand others.

That domain is typically part of the Kochava SDK tracking tools used by various apps.

Which particular app you’re using that is accessing that domain?

Ask your service provider for assistance with whatever mayhem they’ve added into your network, as well.

.xyz is a valid top-level domain, just like .com, .org, .net.

vaicore is the registered name of the company or service that is operating it, which might be legit, but certainly tries to make it non-obvious to uncover what it is doing. It could be a third party service that a certain content provider is using. This site suggests that it might be part of the Paramount+ service. The vaicore.xyz domain has been in use since 2021.