With url /auth/admin/realms/{realmName}/client-scopes you can retrieve all realm scopes.
Then with url /auth/admin/realms/{realmName}/clients/{clientId}/optional-client-scopes optional client scopes and with url /auth/admin/realms/{realmName}/clients/{clientId}/default-client-scopes.
Note: scope could be assigned as default or optional, not both.