I have inherited the following code in a vfp controller class with a hard coded url. I decided to modify it:-

String customDomainName = 'https://toptechservice—partial-c.documentforce.com/sfc/dist/version/renditionDownload?rendition=ORIGINAL_Jpg&versionId=';

Integer index = DistributionPublicUrl.IndexOf('/a/',0);
String strSubString = DistributionPublicUrl.substring(index);

customDomainName = customDomainName + ContentVersionId + '&operationContext=DELIVERY&contentId=' + ContentDocumentId + '&page=0&d=';

customDomainName = customDomainName + strSubString + '&oid=' + orgId + '&dpt=null&viewId=';

system.debug('customDomainName: '+customDomainName);
       
I decided to modify it using URL.getSalesforceBaseUrl() as follows:

String orgDomain = URL.getSalesforceBaseUrl().getHost();
String orgURL = orgDomain.replace('visual','document');

String customDomainName = 'https://'+orgURL+'/sfc/dist/version/renditionDownload?rendition=ORIGINAL_Jpg&versionId=';

Depite both versions appearing identical in the Debug log only the hard-coded version works. When I view the vfp in the browser I can see that the string passed by my modified code has it's domain address changed to the Site URL. The string passed by the hard-coded version is not changed and hence works.

I just want to understand.