You can download data from the sandbox using the files.read() method. 
import fs from 'fs'
import { Sandbox } from 'novita-sandbox/code-interpreter'

const sandbox = await Sandbox.create()

// Create a file in the sandbox for testing
const filePathInSandbox = '/tmp/test-file'
await sandbox.files.write(filePathInSandbox, "test-file-content")

// Read file from sandbox
const content = await sandbox.files.read(filePathInSandbox)

// Write file to local filesystem
const localFilePath = './local-test-file'
fs.writeFileSync(localFilePath, content)

await sandbox.kill()