Transit Gateway supports multiple architecture patterns.
Flat network: All attachments share one route table. Every VPC reaches every other. Simple but no isolation.
Isolated VPCs: Each VPC in its own route table. Routes only to shared services.
Shared services: Shared services VPC in one route table. Others route there only.
Inspection: Traffic routes through a security VPC with firewall first.
Multi-region: Transit Gateway is regional. Peer Transit Gateways across regions.
Draw your architecture before implementing. Routing mistakes isolate or expose environments.