Why is no route table entry needed for AWS Private Link/Interface endpoint?

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My current understanding is, when an instance in a VPC looks to send traffic, it goes through the route table to know where to send that traffic to. So concretely, if I have an instance that wants to connect to a VPC endpoint, what happens? How does it get there?

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For a VPC Endpoint you don't need to edit the route table. The reason for this is that when a VPC Endpoint is created, an ENI (elastic network interface) is placed into your subnet of preference. This means that you will get a private IP address in the range of the subnet, so you wont need to do additional routing configuration.

So concretely, if I have an instance that wants to connect to a VPC endpoint, what happens? How does it get there?

You connect to the endpoint using its DNS or local IP.

In the other hand, if you create a Gateway Endpoint, AWS automatically adds a prefix list to your route table. Obviously there will be no ENI placed in the VPC, and consequently no additional charges will be applied to your account. Gateway Endpoints can be used for S3 and DynamoDB only.