When connecting on-premises networks to AWS using Direct Connect and planning to grow from a few VPCs to hundreds in one Region, which service or feature simplifies and scales connectivity across many VPCs?

AWS Transit Gateway is the correct choice because it acts as a central hub for connecting your on-premises networks to multiple Amazon Virtual Private Clouds (VPCs) and vice versa. This simplifies network architecture and management, especially when scaling to hundreds of VPCs in a single region, by eliminating the need for numerous individual connections. VPC endpoints allow private connectivity to AWS services from within a VPC, not for interconnecting multiple VPCs or on-premises networks. Amazon Route 53 is a highly available and scalable cloud Domain Name System (DNS) web service, not a networking service for interconnecting VPCs. AWS Secrets Manager helps protect access to your applications, services, and IT resources, and is unrelated to network connectivity.