What the registration at
https://app.storj.io exactly(technically) does? Where it stores reg data (emails, passwords) ?
Registration gives you an account on the bridge which gives you access to lots of push button tools to use the network. You are not required to sign up to use the network, but it's 100x easier though the bridge.
The email and double hashed password (once on client, once on server) get stored in the bridge's database, so whoever is running the bridge server has that information.
So essentially, you have the option to choose a bridge you trust, run your own, or go without (in order of convenience vs privacy).