When you intially click on Monitoring(Stackdriver Monitoring) it creates a workspac(a stackdriver account) linked to the ACTIVE(CURRENT) Project from which it was clicked.
Now if you change the project and again click onto Monitoring it would create an another workspace(a stackdriver account) linked to the changed ACTIVE(CURRENT) Project, we don't want this as this would not consolidate our result into a single dashboard(workspace/stackdriver account).
If you have accidently created two diff workspaces merge them under Monitoring > Settings > Merge Workspaces > MERGE.
If we have only one workspace and two projects we can simply add other GCP Project under Monitoring > Settings > GCP Projects > Add GCP Projects.
https://cloud.google.com/monitoring/settings/multiple-projects
Nothing about groups https://cloud.google.com/monitoring/settings?hl=en

When we create subnets in the same VPC with different CIDR ranges, they can communicate automatically within VPC. Resources within a VPC network can communicate with one another by using internal (private) IPv4 addresses, subject to applicable network firewall rules Ref: https://cloud.google.com/vpc/docs/vpc

The Datastore emulator provides local emulation of the production Datastore environment. You can use the emulator to develop and test your application locally Ref: https://cloud.google.com/datastore/docs/tools/datastore-emulator

"Billing Account Viewer access would usually be granted to finance teams, it provides access to spend information, but does not confer the right to link or unlink projects or otherwise manage the properties of the billing account." https://cloud.google.com/billing/docs/how-to/billing-access

Reference:
Default behavior does not grant access to the "your GCP Project" Default behavior allow only create billing account and project - When the organization is created, all users in your domain are automatically granted Project Creator and Billing Account Creator IAM roles at the organization level. This enables users in your domain to continue creating projects with no disruption.