Send Email versus Email Alert
Key Takeaway
Use an Email Alert action to reuse an email alert that is already configured. Use the Send Email action to send to any address or to write content in the flow with merge fields from any object. Also use the Send Email action to attach files or log the email as an activity.
A flow can send email in two main ways. The Send Email action builds the email inside the flow, using recipients and content you set there. An Email Alert action reuses an email alert that was already set up in Setup with a fixed email template and a static list of recipients.
What an email alert is
An email alert is a reusable email made of an email template plus a list of recipients. You create it in Setup under Email Alerts, and you tie it to one object.
A flow, an approval process, or an entitlement process can use an email alert. Legacy workflow rules and Process Builder processes also use email alerts. An alert works the same whichever of these triggers it.