ADP SSL Certificate Usage in Integrations

I have a .pem and .key file provided by ADP. I have uploaded to the Admin Console under "Certificates" --> "Trusted Server Certificates", when I add it to the "Client Certificates" I get a error for missing key file even though I'm not sure how to add it as well since its only looking for a .pem file.

First question: is this correct to add it under the "Trusted Server Certificates" to use in an integration that calls the web service or should it be in "Client Certificates" and if so, how do I add the key?

Now I read in the documentation that you're supposed to create a "Connected System" with HTTPS and set Authentication to "None". But I have a client id and client secret to add but to the body.

Second question: Do I create a Connected System with None Authorization, then create an Integration that adds the client id/client secret to the Body in there?

Third question: how do I utilize the client certificate in the Integration, is there a function that calls it? Or do I add it somehow in the Connected System?

Fourth question: The token needed to be used for the API call, how do I add that in the integration throughout its lifetime?

Thank you all.

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