You can comment using an account on any ActivityPub-compatible instance. On most platforms, you can find the video by typing its URL in the search bar and then comment it from within the software's interface.
We recommend you to not use the <x id="START_TAG_STRONG"/>root<x id="CLOSE_TAG_STRONG"/> user to publish your videos, since it's the super-admin account of your instance. <x id="LINE_BREAK"/> Instead, <x id="START_LINK"/>create a dedicated account<x id="CLOSE_LINK"/> to upload your videos.