Is VPC Peering secure? Do VPC Peering and PrivateLink not use an internet gateway or any other gateway?
VPC Peering allows connectivity between two VPCs. It's just like normal routing between network segments.
VPC PrivateLink allows you to publish an "endpoint" that others can connect with from their own VPC. It's similar to a normal VPC Endpoint, but instead of connecting to an AWS service, people can connect to your endpoint. Think of it as a way to publish a private API endpoint without having to go via the Internet.
In both cases, no traffic goes across the Internet. Connectivity is directly between the VPCs.