Hi @Aaron Nguyen ,
To implement Google Authentication, first we need to get the Google client ID and secret. You can refer to the Google Authentication document and Create the Google OAuth 2.0 Client ID and secret.
Then, depending on whether you are using Asp.net core Identity or not, you can refer the following steps to implement Google Authentication in an MVC application.
If using Asp.net core Identity, since the ASP.NET Core Identity has a built-in support for authentication service that works on OAuth like Google, Facebook, Microsoft, LinkedIn, Twitter, etc. There is no need to add the External login button by ourselves. In this scenario, you can do the following steps:
- Install the
Microsoft.AspNetCore.Authentication.Google package via Nuget.
- Configure the Google Authentication Pipeline.
Open the appsettings.json file and add the Google configuration (in my sample, I'm using appsettings.json file to store the google clientid and secret, you can also refer to the official document to store the values with Secret Manager). Like this:
Open the Program.cs file, add the yellow part of code:
Then, you can find the Google Login button in the Identity Login page (you can use Scaffold to add the Identity pages, refer to Scaffold Identity in ASP.NET Core projects) and login with Google account. 
If you want to implement Google Authentication without Asp.net Identity, you have to add the Google Login button by yourself. Refer to the following sample:
- Create a MVC application without select the "Individual Accounts" Authentication Type.
- Install the
Microsoft.AspNetCore.Authentication.Google package via Nuget.
- Configure the Google Authentication Pipeline.
Still use appsettings.json file to store the Google ClientId and secret.
Add the yellow part of code in the Program.cs file,

- Create an Account controller to add the Google Login and Login Response endpoints:

- Add the Google Login button in the
_Layout.cshtml page:
You can view the relates code from here: 252944-sourcecode.txt
After that, the result as below:

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Best regards,
Dillion