The returnUrl query parameter passed to the login page refers to the URL the prior request came from. This URL typically refers to the IdentityServer authorization endpoint and contains the original request parameters sent from the client. These parameters might contain information your login page needs to customize its workflow. Some examples would be for branding, dynamic page customization (e.g. which external login providers to use), or controlling what credentials the client application expects (e.g. perhaps MFA is required).
In order to read the original authorize request parameter values, you can use the interaction service. It provides a GetAuthorizationContextAsync API that will extract that information from the returnUrl and return a AuthorizationRequest object which contains these values.
It is unnecessary (and discouraged) for your login page logic to parse the returnUrl itself.