I would first of all think how you want to arrange the management groups, subscriptions and resource groups. Are there particular subscriptions and resource groups that go together for example?
Then use Pulumi to describe them. Also, you'll probably want to use the stacks to describe the environment and the project to describe the grouping of resources. More info on stacks here:
https://www.pulumi.com/docs/intro/concepts/stack/
There's also a great article on organising projects and stacks here:
https://www.pulumi.com/docs/guides/organizing-projects-stacks/