In certain scenarios, an opportunistic update is useful because you don't want to cause instability to the system if it can be avoided. For example, if you have a non-critical update that can be applied as necessary without any urgency and you have a MIG that is actively being autoscaled, perform an opportunistic update so that Compute Engine does not actively tear down your existing instances to apply the update. When resizing down, the autoscaler preferentially terminates instances with the old template as well as instances that are not yet in a RUNNING state.

Explanation/Reference:
Explanation:
D: Handling "Unable to connect on port 22" error message
Possible causes include:
There is no firewall rule allowing SSH access on the port. SSH access on port 22 is enabled on all

Compute Engine instances by default. If you have disabled access, SSH from the Browser will not
work. If you run sshd on a port other than 22, you need to enable the access to that port with a custom
firewall rule.
The firewall rule allowing SSH access is enabled, but is not configured to allow connections from GCP

Console services. Source IP addresses for browser-based SSH sessions are dynamically allocated by
GCP Console and can vary from session to session.
F: Handling "Could not connect, retrying..." error
You can verify that the daemon is running by navigating to the serial console output page and looking for
output lines prefixed with the accounts-from-metadata: string. If you are using a standard image but you do
not see these output prefixes in the serial console output, the daemon might be stopped. Reboot the
instance to restart the daemon.
References:
https://cloud.google.com/compute/docs/ssh-in-browser
https://cloud.google.com/compute/docs/ssh-in-browser

Folders are nodes in the Cloud Platform Resource Hierarchy. A folder can contain projects, other folders, or a combination of both. You can use folders to group projects under an organization in a hierarchy. For example, your organization might contain multiple departments, each with its own set of GCP resources. Folders allow you to group these resources on a per-department basis. Folders are used to group resources that share common IAM policies. While a folder can contain multiple folders or resources, a given folder or resource can have exactly one parent.
References:
https://cloud.google.com/resource-manager/docs/creating-managing-folders

Migrating data to Google Cloud Platform
Let's say that you have some data processing that happens on another cloud provider and you want to transfer the processed data to Google Cloud Platform. You can use a service account from the virtual machines on the external cloud to push the data to Google Cloud Platform. To do this, you must create and download a service account key when you create the service account and then use that key from the external process to call the Cloud Platform APIs.
References:
https://cloud.google.com/iam/docs/understanding-service-accounts#migrating_data_to_google_cloud_platform