You can set a single reaction type that applies to every video in your channel, set a different type of reaction for every video in your channel, or a combination of both.
Reactions
To set an overarching or default reaction type for your channel:
Navigate to the Reactions tab of your channel setup
Choose a reaction type from the drop-down menu and customize those reactions using the left side panel
To override default reactions for a single video:
Click on the video you want to adjust to enter its setup
Navigate to the Reactions tab under the video setup page
Use the dropdown to select the reaction type you want that video to have
Note: Video automatically have their reaction type set to "Inherits from Channel" upon upload. Adjusting the channel-level reaction type will update the reaction type of each video that is still set to 'Inherits from Channel' but not videos that have their own reaction type.
Questions
For more information on adding questions to videos, see this article.