if you define google as an idp on the default settings, you have it available on each organization. lets assume you have org a and org b, on org a you are fine with the default configuration, so you don't overwrite something which means the users will just have the google idp, org b you overwrite the configuration, you add an entra id as idp, but you still want to them to be able to use goole so you can also add the idp defined in the default additionaly