I want to know if the URL or website address idmsa.apple.com is a fake or phishing website, or is it a legitimate Apple website?
The URL idmsa.apple.com is legitimate, it is Apple's central sign-in page or portal.

Remember, once the right part of the domain name, which is apple.com is legtimate, everything that follows on the left side (idmsa.apple.com) is also legitimate.