To deploy a Flutter Web app to Amazon S3 and Bitrise, you need to create an Amazon S3 bucket and configure it as a static web site. You can use the AWS Free Tier to get started with 5GB of storage, 20,000 Get Requests, and 2,000 Put, Copy, Post, or List Requests. Once you've created your S3 bucket, you can host a static website by creating an index document and enabling static website hosting. To deploy your Flutter Web app to Amazon S3 via Bitrise, you need to add the AWS Access Key and Private Key as secret environment variables, add a Flutter Step to build the web application, and add an Amazon S3 Bucket Sync step to upload the local folder to the S3 bucket. After triggering the build, you can refresh the S3 bucket and click on the index.html page to see your app deployed. Additionally, you can use Amazon CloudFront to serve HTTPS requests for your Amazon S3 bucket if you want to use HTTPS.