Service Gateway is virtual router that you can add to your VCN. It provides a path for private network traffic between your VCN and supported services in the Oracle Services Network like Object Storage) so compute Instances in a private subnet in your VCN can back up data to Object Storage without needing public IP addresses or access to the intern

Autoscaling lets you automatically adjust the number of Compute instances in an instance pool based on performance metrics such as CPU utilization. This helps you provide consistent performance for your end users during periods of high demand, and helps you reduce your costs during periods of low demand.
you can associate a load balancer with an instance pool. If you do this, when you add an instance to the instance pool, the instance is automatically added to the load balancer's backend set . After the instance reaches a healthy state (the instance is listening on the configured port number), incoming traffic is automatically routed to the new instance.
Instance pools let you provision and create multiple Compute instances based off the same configuration, within the same region.
By default, the instances in a pool are distributed across all fault Domains in a best-effort manner based on capacity. If capacity isn't available in one fault domain, the instances are placed in other fault domains to allow the instance pool to launch successfully.
In a high availability scenario, you can require that the instances in a pool are evenly distributed across each of the fault domains that you specify. When sufficient capacity isn't available in one of the fault domains, the instance pool will not launch or scale successfully, and a work request for the instance pool will return an "out of capacity" error. To fix the capacity error, either wait for capacity to become available, or use the UpdateInstancePool operation to update the placement configuration (the availability domain and fault domain) for the instance pool.
during create the instance pool you can select the location where you want to place the instances" In the Availability Domain list, select the availability domain to launch the instances in.
If you want the instances in the pool to be placed evenly in one or more fault domains, select the Distribute instances evenly across selected fault domains check box. Then, select the fault domains to place the instances in.

MongoDB is a cross-platform document-oriented database program. Classified as a NoSQL database program, MongoDB uses JSON-like documents with schema, so the best to be migrated to Oracle NoSQL Database.
https://blogs.oracle.com/nosql/migrate-mongodb-data-to-oracle-nosql-database Autonomous transaction Processing Serverless (ATP-S) isn't supported yet for EBS database

Explanation
Autonomous Database is an Oracle Managed and Secure environment.
A physical database can't simply be migrated to autonomous because:
- Database must be converted to PDB, upgraded to 19c, and encrypted
- Any changes to Oracle shipped privileges, stored procedures or views must be removed
- All legacy structures and unsupported features must be removed (e.g. legacy LOBs) GoldenGate replication can be used to keep database online during migration

Explanation
Pre-authenticated requests provide a way to let users a bucket or an object without having their own credentials, as long as the request creator has permission to access those objects.
For example, you can create a request that lets operations support user backups to a bucket without owning API keys. Or, you can create a request that lets a business partner update shared data in a bucket without owning API keys.
When creating a pre-authenticated request, you have the following options:
You can specify the name of a bucket that a pre-authenticated request user has write access to and can upload one or more objects to.
You can specify the name of an object that a pre-authenticated request user can read from, write to, or read from and write to.
Scope and Constraints
Understand the following scope and constraints regarding pre-authenticated requests:
Users can't list bucket contents.
You can create an unlimited number of pre-authenticated requests.
There is no time limit to the expiration date that you can set.
You can't edit a pre-authenticated request. If you want to change user access options in response to changing requirements, you must create a new pre-authenticated request.
The target and actions for a pre-authenticated request are based on the creator's permissions. The request is not, however, bound to the creator's account login credentials. If the creator's login credentials change, a pre-authenticated request is not affected.
You cannot delete a bucket that has a pre-authenticated request associated with that bucket or with an object in that bucket.