We are using Sitefinity 10 which comes with this package:
Can we replace that with the actual code for Identity Server 3 for customization and debugging purposes?
How much was that code changed by Progress for inclusion in the package they are hosting?
What is your ultimate goal? You can get the SitefinityWebApp pdb files from the Sitefinity/Telerik portal.
I would look into the provider/manager model of sitefinity for customization. docs.sitefinity.com/authentication-and-single-sign-on-(sso)