Hey @Simone Sauter50! Whether existing subscribers in that sequence will receive that email actually depends on how they were added to the sequence. We have a guide on this here: Will My Existing Subscribers Receive New Emails Added to My Sequence?128
But to answer your other question, any new sequence emails would need to be added to the sequence itself. Automations connect different things together — for example, if you want to have your subscribers get added to a sequence after they opt in to a form/landing page, then you'd have to first create the form/landing page from the forms page44, then create the sequence from the sequences page49, then you could connect those two together (i.e. automate them so that signing up to the form → adds them to the sequence) via a visual automation48 or automation rule46.
For further insight on which subscribers will/will not receive your newly added sequence email, please feel free to reach out to our team at firstname.lastname@example.org (or via the in-app messenger in your account) with the name of the sequence & email :) One of our Product Specialists will be able to take a look at it in your account and provide a more specific answer from there.