I am new to working with APIs, and I recently started a Python project making use of the Google Calendar API that has been put on github. As such, to protect the API keys I created a .env file and stored the keys as environment variables.
I followed guides that told me to make sure to .gitignore the .env file. However, I don't understand how a user who downloads my app and uses it would be able to access the API key values in the .env file, if the .env file is not in the git repo to begin with.
The values in my .env file are essential to authorizing the user's Google account (via OAuth) for use with the app.
What steps would I take to make sure a user of the app is able to retrieve the variables in the ignored .env file?
I see, so I would need to provide the values to them somehow, and have them configure it manually?

Yes, but if those values are sensitive, there should not be stored in the Git repository in the first place.
Which means your README (in that git repository) should include instructions in order for a user to:

fetch those values
build the env file
How would a user who installs my app access variables from the .env file, if it is ignored by git?