Use System.Url.getOrgDomainUrl() to get the Salesforce domain URL using Apex . 

Use getOrgDomainUrl() in orgs with or without My Domain to retrieve canonical URLs. For example: https://yourDomain.my.salesforce.com, or, for orgs without My Domain enabled https://yourInstance.salesforce.com.

Sample Code:

System.debug(
    'Domain URL is '+
    System.Url.getOrgDomainUrl() 
);

Output: