Hello Blockstackies!!!
Hope you are doing Excellent!!
I want to know that, which key is used for uploading profile.json into gaia hub?
Also tell when the App_key comes into picture and Using only one Owner key can i make multiple owner address?
For uploading profile.json a derived key from your seed phrase (=master/root secret) is used, more precisely following bip39 standard, it is path: m/‘888’/0’ + index
If a username is registered with this key, then this key is the owner key of the username/blockstack ID.
You can derive more keys by increasing the index by 1. Thereby, you can have multiple owner addresses (= multiple blockstack IDs) using the one master secret.
App keys come into the picture when an app asks the user to sign in. Then the domain name of the app is used to derive another key from the master secret. This keys is handed over to app for accessing the sandboxed storage bucket for this user for this app.