A domain is defined by a release configuration. In order to define a domain, you need to create a new release config yaml file in your repository
A simple release config can be defined as shown below
// Sample release config <business_domain.yaml>releaseName:<business_domain># --> The name of the domainpool:<sandbox/scratch org pools>excludeAllPackageDependencies:trueincludeOnlyArtifacts:# --> Insert packages-<pkg1>releasedefinitionProperties:promotePackagesBeforeDeploymentToOrg:prod
The resulting file could be stored in your repository and utilized by the commands such as build, release etc.