Oracle Cloud Infrastructure is hosted in regions and availability domains. A region is a localized geographic area, and an availability domain is one or more data centers located within a region. A region is composed of one or more availability domains. Most Oracle Cloud Infrastructure resources are either region-specific, such as a virtual cloud network, or availability domain-specific, such as a compute instance. Traffic between availability domains and between regions is encrypted. Availability domains are isolated from each other, fault tolerant, and very unlikely to fail simultaneously. Because availability domains do not share infrastructure such as power or cooling, or the internal availability domain network, a failure at one availability domain within a region is unlikely to impact the availability of the others within the same region.
The availability domains within the same region are connected to each other by a low latency, high bandwidth network, which makes it possible for you to provide high-availability connectivity to the internet and on-premises, and to build replicated systems in multiple availability domains for both high-availability and disaster recovery.
Oracle is adding multiple cloud regions around the world to provide local access to cloud resources for our customers. To accomplish this quickly, we've chosen to launch regions in new geographies with one availability domain.
As regions require expansion, we have the option to add capacity to existing availability domains, to add additional availability domains to an existing region, or to build a new region. The expansion approach in a particular scenario is based on customer requirements as well as considerations of regional demand patterns and resource availability.
For any region with one availability domain, a second availability domain or region in the same country or geo-political area will be made available within a year to enable further options for disaster recovery that support customer requirements for data residency where they exist.

Cloud Horizontal Scaling refers to provisioning additional servers to meet your needs, often splitting workloads between servers to limit the number of requests any individual server is getting. In a cloud-based environment, this would mean adding additional instances instead of moving to a larger instance size.
Cloud Vertical Scaling refers to adding more CPU or memory to an existing server, or replacing one server with a more powerful server.
Reference:
https://cloudcheckr.com/cloud-cost-management/cloud-vs-data-center-what-is-scalability-in-cloud-computing/ Horizontal scaling means that you scale by adding more machines into your pool of resources whereas Vertical scaling means that you scale by adding more power (CPU, RAM) to an existing machine.
An easy way to remember this is to think of a machine on a server rack, we add more machines across the horizontal direction and add more resources to a machine in the vertical direction.

With horizontal-scaling it is often easier to scale dynamically by adding more machines into the existing pool - Vertical-scaling is often limited to the capacity of a single machine, scaling beyond that capacity often involves downtime and comes with an upper limit.

Resource Manager is an Oracle Cloud Infrastructure service that allows you to automate the process of provisioning your Oracle Cloud Infrastructure resources. Using Terraform, Resource Manager helps you install, configure, and manage resources through the "infrastructure-as-code" model.
A Terraform configuration codifies your infrastructure in declarative configuration files. Resource Manager allows you to share and manage infrastructure configurations and state files across multiple teams and platforms. This infrastructure management can't be done with local Terraform installations and Oracle Terraform modules alone. For more information about the Oracle Cloud Infrastructure Terraform provider, see Terraform Provider. For a general introduction to Terraform and the "infrastructure-as-code" model, see https://www.terraform.io.

In OCI a budget can be used to set soft limits on your Oracle Cloud Infrastructure spending. You can set alerts on your budget to let you know when you might exceed your budget, and you can view all of your budgets and spending from one single place in the Oracle Cloud Infrastructure console.
Budgets are set on
* Cost-tracking tags
* Compartments (including the root compartment)
Reference:
https://docs.cloud.oracle.com/en-us/iaas/Content/Billing/Concepts/budgetsoverview.htm