Google play app signing process certificates

I am trying to understand the signing process with the Play App Signing. Hence couple of qquestions.

1. I am using goole maps API, why I need to add to API SHA-1 certificate fingerprint from App signing key certificate on the console? Why I cant that one in the local KEY file that is no my pc?
2. What I need to do with the provate_key.pepk that I generated from release?
3. What is the purpose of deployment_cert.der?
4. What is the purpose of upload_cert.der?

Can you please explain me the correct process of signing an app and how to use it?

Also at: https://support.google.com/googleplay/android-developer/answer/9842756#create&zippy=%2Cexisting-apps

Existing apps->Step 2->5 Syas: Select the export and upload option that best suits your release process and upload an existing app signing key.

I do not see an option for upload an existing app signing key...

from Google play app signing process certificates