I like Transit Gateway on so many levels, truly an NG service integrating many different points of ingress in a way with VPCs

Few important points to start with

1. AWS Transit Gateway is a service that enables customers to connect their Amazon Virtual Private Clouds (VPCs) and on-premises networks to a single gateway.
2. Transit Gateway is a hub that controls traffic routed among all the connected networks.
3. Transit Gateway supports both IPv4 and IPv6 traffic.
4. Transit Gateway is highly scalable and can support thousands of VPCs and on-premises networks.
5. Transit Gateway uses route tables to determine how traffic is routed.
6. Transit Gateway supports VPC peering and VPN connections.
7. Transit Gateway can be used with AWS Direct Connect to create a private connection between an on-premises network and a VPN

Scenario 1 - Connect your VPCs

Interconnecting VPCs’s typically done through VPC-Peering, now while that is still valid you can easily interconnect VPCs through the transit gateway attachments feature, while VPC peering does only well VPC, transit gateway can connect VPCs, DX-Gateways and you can terminate IPSEC-VPN’s directly onto the transit gateway.

• Routing tables are not auto-propagated, meaning you have to add static routes individually in the VPCs
• In the case of the Transit gateway attachment, there is an additional effort to set up the connection and then set up the attachment followed by adding routes to the remote-routing table.