Background
I have a google sheet, who's data I need to process on my local system. The nature of processing is very tedious and long so I wrote a script for it.
Problem
I need to access the google sheet through the python script to process it further. Online it is mentioned that to read the private google sheet directly, I need to create a GCP project and within that project, I need to create a service account. After that I should download the credentials and share the google sheet with that service account email.
Two problems here are :

Downloading the credentials -- insecure and my organization prohibits it.
Sharing of google sheet with service account email. organization also prohibits sharing sheet with outside organization emails.

What I found as a solution
I came across a solution of impersonating a service account but I could not find anything as to how can I do that. (Would appreciate any insights on that). All the other solutions suggested to download credentials which is a big NO.
For the sharing of sheets thing, I guess we can use drive API, but same problems are with that.
I tried using gcloud auth login and gcloud auth application-default login but was getting errors

Request had insufficient authentication scopes.". Details: "Insufficient Permission: Request had insufficient authentication scopes.

using ['https://www.googleapis.com/auth/spreadsheets',  https://www.googleapis.com/auth/drive'] as scopes
What I need? (Summary)
How to access google sheets API (or download the Sheet from google drive) without downloading any sort of credentials.json.
problem
Your only option to access private user data is to be authorized as a user who can access the file. Either logging in using Oauth2 or using a service account

Downloading the credentials -- insecure and my organization prohibits it.

In order to use a google api you must first register your application on google developer console and download the client credentials then a user must authorize the application using Oauth2, which would mean both downloading the credentials.json file from google developer console and you getting user token credentials from the authorized user.

Sharing of google sheet with service account email. organization also prohibits sharing sheet with outside organization emails.

In order to use a service account you would again need to first register your application on google developer console and download the client credentials for the service account.  Then you would need to share the sheet with the service account.
service account impersonation.
Service account impersonation is used by GSuite domain. You create a normal service account again downloading the credentials, and then the GSuite admin is able to delegate authority to the service account which will allow it to impersonate a user on the domain.
This would require you to have GSuite domain and the owner of the file being also on the GSuite domain, and you creating a project on google developer console and downloading the credentials.json for the service account, which you already stated you couldn't do.
conclusion
I guess what i am saying is there is no way with the limitations imposed by your organization for you to access a private google sheet or any private user data on googles system, via any api.
How to read private Google Sheets using Google Sheets API without service account or downloading credentials