//To find Salesforce Base URL
//Till API Version 58.0
system.debug('@@@@' + URL.getSalesforceBaseUrl());
//After API Version 58.0
system.debug('@@@@' + URL.getOrgDomainUrl()); //Canonical URL
system.debug('@@@@ ' + URL.getCurrentRequestUrl()); //URL of an entire request
For more information, please check the URL Methods.