As far as I understand (I may be wrong), it doesn't get sent to Steemit. JavaScript in the browser uses what you input there, runs the appropriate signing algorithm to create signed content and then sends that signed content to Steemit. Steemit then uses the public key to validate the signature. If you're familiar with public and private key encryption and signing / validating signatures, this makes a lot more sense. PGP is a great example and I've used that for a long time so it's familiar to me. I hope that helps. Thanks for asking these questions!