An automation used to start on one event. Now it can start on as many as you like.
On the automation editor, press Add Another Start Trigger. Each row is its own trigger with its own settings, for example: joins the Buyers list, OR gets the tag vip, OR clicks a link containing your sales page, OR makes a purchase.
Any one of them starts the flow. They are OR, not AND. That is deliberate: these are events, and two events happen at different moments, so nobody can join a list and click a link in the same instant.
When you want to require two things at once, that is a condition, not a trigger. Start the automation on the event, then make the first step an If / Else that checks the other thing. For example: start when they join any list, then If they have the tag customer, end the flow, otherwise send the promo. That is how you exclude existing buyers from a pitch.
The list and form pickers are searchable, so a site with thousands of lists is no problem. Just start typing the name.