HTTP(S) load balancing is a Google-recommended practice for distributing web traffic across multiple regions and zones, and providing high availability, scalability, and security for web applications. It supports both IPv4 and IPv6 addresses, and can handle SSL/TLS termination and encryption. It also integrates with Cloud CDN, Cloud Armor, and Cloud Identity-Aware Proxy for enhanced performance and protection. A MIG can be used as a backend service for HTTP(S) load balancing, and can automatically scale and heal the VM instances that host the web application.
To configure DNS for HTTP(S) load balancing, you need to create an A record in your DNS public zone with the load balancer's IP address. This will map your domain name to the load balancer's IP address, and allow users to access your web application using the domain name. A CNAME record is not recommended, as it can cause latency and DNS resolution issues. A private zone is not suitable, as it is only visible within your VPC network, and not to the public internet.
HTTP(S) Load Balancing documentation
Setting up DNS records for HTTP(S) load balancing
Choosing a load balancer

Create an instance template for the instances so VMs have same specs. Set the "˜Automatic Restart' to on to VM automatically restarts upon crash. Set the "˜On-host maintenance' to Migrate VM instance. This will take care of VM during maintenance window. It will migrate VM instance making it highly available Add the instance template to an instance group so instances can be managed.
* onHostMaintenance: Determines the behavior when a maintenance event occurs that might cause your instance to reboot.
* [Default] MIGRATE, which causes Compute Engine to live migrate an instance when there is a maintenance event.
* TERMINATE, which stops an instance instead of migrating it.
* automaticRestart: Determines the behavior when an instance crashes or is stopped by the system.
* [Default] true, so Compute Engine restarts an instance if the instance crashes or is stopped.
* false, so Compute Engine does not restart an instance if the instance crashes or is stopped.
Enabling automatic restart ensures that compute engine instances are automatically restarted when they crash.
And Enabling Migrate VM Instance enables live migrates i.e. compute instances are migrated during system maintenance and remain running during the migration.
Automatic Restart If your instance is set to terminate when there is a maintenance event, or if your instance crashes because of an underlying hardware issue, you can set up Compute Engine to automatically restart the instance by setting the automaticRestart field to true. This settingdoes not apply if the instance is taken offline through a user action, such as calling sudo shutdown, or during a zone outage.
Ref: https://cloud.google.com/compute/docs/instances/setting-instance-scheduling-options#autorestart Enabling the Migrate VM Instance option migrates your instance away from an infrastructure maintenance event, and your instance remains running during the migration.Your instance might experience a short period of decreased performance, although generally, most instances should not notice any difference. This is ideal for instances that require constant uptime and can tolerate a short period of decreased performance.
Ref: https://cloud.google.com/compute/docs/instances/setting-instance-scheduling-options#live_migrate

You can deploy to a different project by using -project flag.
By default, the service is deployed the current project configured via:
$ gcloud config set core/project PROJECT
To override this value for a single deployment, use the -project flag:
$ gcloud app deploy ~/my_app/app.yaml -project=PROJECT
Ref: https://cloud.google.com/sdk/gcloud/reference/app/deploy