What is the desktop application URL format msal<client_id>://auth

What is the 1st one (msal<client_id>://auth) ?

msal<client_id>://auth is a Desktop/Mobile Redirect URI(Reply URL) is the location where the authorization server sends the user once the app has been successfully authorized and granted an authorization code or access token. The authorization server sends the code or token to the redirect URI, so it's important you register the correct location as part of the app registration process. For more information you can follow this MS Document in which format and Maximum number of Redirect URI can be do.

What standard does its format coming from?

When you are adding a Redirect URL for Mobile+Desktop application. You don't need to add a redirect URI (if you're building a Xamarin Android and iOS application) that doesn't support the broker redirect URI. It is automatically set to msal{ClientId}://auth for Xamarin Android and iOS.

You can override the redirect URI by using the RedirectUri property (for example, if you use brokers). Here are some examples of redirect URIs for that scenario:

• RedirectUriOnAndroid = "msauth-5a43469XXXXXc://com.microsoft.identity.client.sample";

• RedirectUriOnIos = $"msauth.{Bundle.ID}://auth";

Please refer this MS document for more information.