For Flask, follow this tutorial. It's the absolute simplest I've come across: https://cloud.google.com/run/docs/quickstarts/build-and-depl...
For React, follow this tutorial: https://docs.aws.amazon.com/AmazonS3/latest/userguide/Hostin...
But instead of the files it tells you to create, just use create-react-app, and run "npm run build". Then take the directory it creates (hint: it's named "build"), and drop all it's contents in your s3 bucket