All Mailchimp plugin tutorials:
Before you start reading this article, please make sure you read this.
In this article, we are assuming you have already followed this and created the lists that would trigger the reminders in your Mailchimp "Campaign".
To find all the segmenting options, go to your event editor > "Plugins" > "Mailchimp Plugin" > View
Let's say you want to add people who attended your webinar to a "List"/"Audience", to do so, just follow this.
All the other options work the same way. Here is another for those who didn't attend webinar.
You can also use that "List/Audience" to send people an email inside Mailchimp "Campaign".