To send an automatic email to your customers, for example, after they sign up for your newsletters, you can use Shopify's official Email app and Flow app .
https://apps.shopify.com/shopify-email
https://apps.shopify.com/flow
And you can create a new automation as seen in the screenshot below.
Also, another solution, which i fact was used before as the standard way before Shopify released its official Flow and Email apps, is to use an external app like Klaviyo.
https://apps.shopify.com/klaviyo-email-marketing
If you have further questions regarding this process, please reply to this thread
