If the build folder exists on the container then you can programmatically read/write to and from it, even if it’s not shown in the file tree. There’s more about that here: https://glitch.com/faq#hidden-files. You can also use the console to interact with it, which is available under Advanced Options from the top-left project info menu.
You can add a watch.json file to your project to change the Glitch restart behaviour, if you want to make use of create-react-apps reloading. There’s one already in the referenced sample app and more detail about how to use it here: https://glitch.com/edit/#!/watch-json
There are size restrictions on the size of files you can copy/paste, see https://glitch.com/faq#restrictions