Hey @TUng 

Series allows you to create a flow which you can insert multiple blocks to send a number of Proactive Support messages (e.g. Posts, Chats, Banners, Emails...) to your users in a sequence. Moreover, you can also insert a Workflow in a Series. You can read more about the Series here.

Whereas Workflows are used to create chatbots which can talk to your customers n a certain logic, or to automate repetitive tasks (E.g. assign inbound message to a certain team/teammates based on a logic you set). You can read more about Workflows here.

For your scenario, as you are trying  to build a chatbot, you can use the Workflow option, but you will be able to embed videos in the first reply though like mentioned here.

If you use Workflow, you need to make sure you are using the right trigger type and audience rules, so the Workflow can trigger when a user exits that certain page on your platform. For your case, I’d recommend using the “Customer visits a page” trigger and then use the “referrer URL” attribute (this attribute hold the URL of the previous page the user was on). So when the user exists that page to navigate another page, their referral URL will hold the URL of the page they exited, You can find more info on triggers settings here.

Hope this helps!