You don’t ‘upload’ the extension to a channel. You upload it to Twitch’s CDN, and you would add the channel your testing it on to the Testing Account Allowlist, and the channel you’re making it for to the Streamer Allowlist.
This will allow you to install the extension on your test channel for the purpose of testing, and the review process, and if it passes review and you set the extension to Live, the extension will then be available to the channel on the Streamer Allowlist