"Google Cloud VPC Network Peering allows internal IP address connectivity across two Virtual Private Cloud (VPC) networks regardless of whether they belong to the same project or the same organization."
https://cloud.google.com/vpc/docs/vpc-peering
while
"Cloud Interconnect provides low latency, high availability connections that enable you to reliably transfer data between your on-premises and Google Cloud Virtual Private Cloud (VPC) networks."
https://cloud.google.com/network-connectivity/docs/interconnect/concepts/overview and
"HA VPN is a high-availability (HA) Cloud VPN solution that lets you securely connect your on-premises network to your VPC network through an IPsec VPN connection in a single region."
https://cloud.google.com/network-connectivity/docs/vpn/concepts/overview

App engine is a regional service, which means the infrastructure that runs your app(s) is located in a specific region and is managed by Google to be redundantly available across all the zones within that region. Once an app engine deployment is created in a region, it cant be changed. The only way is to create a new project and create an App Engine instance in europe-west3, send all user traffic to this instance and delete the app engine instance in us-central.
Ref: https://cloud.google.com/appengine/docs/locations

https://cloud.google.com/vpc/docs/vpc#manually_created_subnet_ip_ranges

Understanding IAM custom roles
Key Point: Custom roles enable you to enforce the principle of least privilege, ensuring that the user and service accounts in your organization have only the permissions essential to performing their intended functions.
Basic concepts
Custom roles are user-defined, and allow you to bundle one or more supported permissions to meet your specific needs. Custom roles are not maintained by Google; when new permissions, features, or services are added to Google Cloud, your custom roles will not be updated automatically.
When you create a custom role, you must choose an organization or project to create it in. You can then grant the custom role on the organization or project, as well as any resources within that organization or project.
https://cloud.google.com/iam/docs/understanding-custom-roles#basic_concepts

https://cloud.google.com/kubernetes-engine/docs/how-to/cluster-
autoscaler#adding_a_node_pool_with_autoscaling