Explanation
A mount target is an NFS endpoint that lives in a VCN subnet of your choice and provides network access for file systems. The mounttarget provides the IP address or DNS name that is used together with a unique export path to mount the file system. A single mount target can export many file systems. Typically, you create your first mount target and export when you create your first file system. The mount target maintains an export set which contains all of the exports for its associated file systems.
Limitations and Considerations
Eachavailability domain is limited to two mount targets by default. However, you can export up to 100 file systems through each mount target.
See Service Limits for a list of applicable limits and instructions for requesting a limit increase.
Each mount target requires three internal IP addresses in the subnet to function. Two of the IP addresses are used during mount target creation. The third IP address must remain available for the mount target to use for high availability failover.
The File Storage service doesn't "reserve" the third IP address required for high availability failover. Use care when designing your subnets and file systems to ensure that sufficient IP addresses remain available for your mount targets.

Explanation
References:https://cloud.oracle.com/database/faq#backup
Can I set up Data Guard across Availability Domains?Yes, you can set up Data Guard in the same or different Availability Domains in a region. However, Oracle recommends that you set up your Data Guard configuration across Availability Domains.Can I set up Data Guard across Oracle Cloud Infrastructure regions?Yes, you can set up Data Guard across regions,
"but the Database Cloud Service Data Guard feature currently does not support it. " You can manually set up Data Guard across regions by logging on to your host and using DGMGRL. You must enable an internet gateway on the primary and standby DB system VCN for Data Guard to transport logs across regions. Learn more about DGMGRL.
To configure a Data Guard system across regions or between on-premises and Oracle Cloud Infrastructure DB systems, you must access the database host directly and use the DGMGRL utility.
https://docs.cloud.oracle.com/iaas/Content/Database/Tasks/usingdataguard.htm

Explanation
References:
The best practice for high availability and durability is to run the primary, standby, and observer in separate availability domains. The observer determines whether or not to failover to a specific target standby database
https://docs.cloud.oracle.com/en-us/iaas/Content/Database/Tasks/usingDG.htm#ConfiguringObserverOptional

Explanation

Explanation
When you create a subnet, by default it's considered public, which means instances in that subnet are allowed to have public IP addresses. Whoever launches the instance chooses whether it will have a public IP address.
You can override that behavior when creating the subnet and request that it be private, which means instances launched in the subnet are prohibited from having public IP addresses. Network administrators can therefore ensure that instances in the subnet have no internet access, even if the VCN has a working internet gateway, and security rules and firewall rules allow the traffic.
There are two optional gateways (virtual routers) that you can add to your VCN depending on the type of internet access you need:
Internet gateway: For resources with public IP addresses that need to be reached from the internet (example:
aweb server) or need to initiate connections to the internet.
NAT gateway: For resources without public IP addresses that need to initiate connections to the internet (example: for software updates) but need to be protected from inbound connections from the internet.
Just having an internet gateway alone does not expose the instances in the VCN's subnets directly to the internet. The following requirements must also be met:
The internet gateway must be enabled (by default, the internet gateway is enabled upon creation).
The subnet must be public.
The subnet musthave a route rule that directs traffic to the internet gateway.
The subnet must have security list rules that allow the traffic (and each instance's firewall must allow the traffic).
The instance must have a public IP address.