What are the basics of Salesforce DX — scratch orgs, project structure, source tracking, and push/pull commands?

Salesforce DX (Developer Experience) is a source-driven development model using the Salesforce CLI.
Scratch orgs: Temporary, configurable orgs created from a project definition file (project-scratch-def.json). They are disposable (1–30 day lifespan), not copies of production, and designed for isolated feature development and automated testing in CI/CD pipelines.
Project structure: A DX project has a sfdx-project.json file defining the namespace and source paths. Metadata lives in the force-app/main/default directory, organised in subfolders per metadata type (objects, classes, lwc, flows, etc.).
Source tracking: Scratch orgs track which local files have changed vs the org and vice versa. This enables precise push/pull of only changed components.
Commands:
— sf project deploy start --target-org scratchOrgAlias: Push local source to the org.
— sf project retrieve start --target-org scratchOrgAlias: Pull org changes back to local source.
— sf org create scratch --target-dev-hub DevHub --definition-file project-scratch-def.json --alias myOrg: Create a new scratch org.