Hello @suchi_klavier,
If you haven’t already, I would suggest taking a look at our Understanding the double opt-in process Help Center article.
As mentioned in it, double opt-in is set on the list level and is enabled by default. Because double opt-in is on a list level, you could have lists that have double opt-in enabled and lists that do not have it enabled. Only subscribers who signup to be added to a list that has double opt-in enabled will receive the confirmation email. For example, if you have a signup form linked to a list named Newsletter that has it enabled, then they’ll need to confirm their subscription prior to being fulled added to the list.
Since double opt-in is already automated, you would not need to setup any extra flows in order for it to trigger. If you would like to customize the confirmation email to be more on brand, you can do so by navigating to the list’s Subscribe & Preference Pages settings and customizing the Email Confirmation page. You can learn more about this process and the various subscribe and preference pages option from the Getting started with consent pages article.
I would also encourage you to take a look at other discussions on this topic in the Community that could be helpful. I’ve included some below that I’ve personally found helpful:
I hope this helps!
David