Where to deploy my reactjs application?

In my case I recommend you using Amplify.

**Here is why using Amplify: **

• It automatically creates CI/CD pipeline that can detect changes in your source repository, then deploy newer version based on your code.

• You can also easily create custom domain with Amplify.

• Simple steps to follow

• Build in authentication component (work with Cognito)

Hello.

For small applications, I think CloudFront + S3 is fine.
https://docs.aws.amazon.com/prescriptive-guidance/latest/patterns/deploy-a-react-based-single-page-application-to-amazon-s3-and-cloudfront.html

I think Amplify is useful when creating a web service that performs authentication using Cognito, etc., because it allows easy integration.
https://docs.aws.amazon.com/amplify/latest/userguide/welcome.html

I think you may consider using App Runner if you are managing web applications in containers.
https://docs.aws.amazon.com/apprunner/latest/dg/what-is-apprunner.html

Hello, As already mentioned in above answers that S3+Cloudfront for small application is fine.

For you code base for React and Nextjs or any other language , using codecommit is a seamless option.

To use custom domain, it works well with Public Hosted Zone and it uses it own custom certificate under the hood. https://docs.aws.amazon.com/amplify/latest/userguide/custom-domains.html