It appears that I need to include an Email Template ID. I can't find an ID anywhere. Also, when I select to include the Email Template ID, then the Recipient ID is also required. There is not 1 individual Recipient for this Flow. Maybe I'm doing it all wrong.
Flows can be daunting to start with, but going to the community is a great way to learn more! @Sarvendra Kumar Singh's answer is on the right track but references apex code rather than flow, so it's unlikely it will help you much. To get the Email Template Id, you should do as the above answer suggests but use a GetRecords element to get the Template Id.
As the documentation states, when you specify an email template, you must also specify a recipient Id (as the email template might have merge fields specific to 1 singular user). As such, you should either use the Send email core action in a loop, using a different Recipient Id each time for each user you need to send to, or move this to multiple transactions.
