Declarative development is the process of creating applications using point-and-click tools and configuration settings, without writing code. To build out the database layer for a Salesforce application, declarative developers can use the following options:
* Roll-Up Summaries: These are fields that calculate values from related records, such as the total amount of closed opportunities for an account. Roll-up summaries can be created on master-detail or lookup relationships, and can use filters to specify which records to include in the calculation1.
* Relationships: These are the links between objects that allow data to be shared and accessed across different objects. There are two types of relationships: master-detail and lookup. Master-detail relationships create a parent-child relationship between two objects, where the master record controls certain behaviors of the detail record, such as security and deletion. Lookup relationships create a loose association between two objects, where the lookup field can be optional or required, and the related record can have its own security and owner2.
* Custom Objects and Fields: These are the basic building blocks of any Salesforce application. Custom objects are the tables that store data specific to the organization or the application. Custom fields are the columns that store the attributes of each record in the object. Custom objects and fields can be created and modified using the Object Manager in Setup3.
References:
* 1: Define Roll-Up Summary Fields
* 2: Create Object Relationships
* 3: Create Custom Objects and Fields

Scratch orgs are short-term Salesforce environments that you can spin up to work on a feature. They are source-driven, disposable, and fully configurable. To create scratch orgs, you need to use the Salesforce CLI, which is a command-line tool that lets you interact with Salesforce orgs and perform various tasks. You can use the Salesforce CLI to create a scratch org definition file, which specifies the features and settings of the scratch org, and then run the force:org:create command to create the scratch org. You can also use the force:org:open command to open the scratch org in a browser without logging in.
References: The use of Salesforce CLI to create scratch orgs can be referenced in the Salesforce Platform Developer I learning documents, specifically in modules that cover Salesforce DX and scratch orgs, such as:
* Get Started with Salesforce DX
* Scratch Orgs
* Develop in Scratch Orgs