Connect your public cloud account with the on-prem deployment of NSX-T Data Center,
Connect VPCs/VNets with on-prem using suitable methods, such as Direct Connect for AWS or Express Route for Microsoft Azure. You can also use site-to-site VPN with any VPN endpoint on-prem and PCG acting as the VPN endpoint in your public cloud.
To use the Transit/Compute topology, you must have peering connections established between the Transit and Compute VPCs/VNets. You can have a single PCG manage multiple compute VPCs/VNets. You can also have a flat compute VPC/VNet architecture with a PCG pair installed in each VPC/VNet. See NSX Public Cloud Gateway: Architecture and Modes of Deployment for details on PCG deployment options.