Good Afternoon,
Not sure I quite understand the use case entirely but, I can attempt to assist.
If your goal is to send an email to a user or a customer for example with a template built into it accessed from Power Apps you could just create a gallery to select your templates, then once a template is selected have it filled into a text input field so that the user can add what they need into the message. Then once they do this allow them to select an email whether you source it from another DataSource(list, office365) or have them natively type it is fine but, then click the button to send from within the app. If you want the email sent from a specific mailbox you can set your run only user permissions from the Power Automate Flow Management screen.
So step-by-step:
1. Connect your list with the different templates
2. Create a gallery to select which template the user wants to use
3. Connect your Office365Users Connector (allow you to dynamically select a recipient)
4. Connect your Office365Outlook Connecor
5. Create a text input field that is filled with currentItem.Template ( However you are connected to the selected item in the Gallery and its template in it )
6. Create a button with the
Office365Outlook.SendEmailV2(Recipient,Subject, Body)
For your values
7. Recipient can be a dropdown field with the office365users.allusers
In my understanding you cannot have the text copy in the Power App and automatically appear in an Email(in outlook). That is why I didn't follow that process.
8. Add a gallery with a similar statment and a textinput for a search bar for the following
Office365Users.SearchUserV2({searchTerm:TextInput1.Text,top:5}).value
So just to rehash at this point a user can: Use Gallery1 to look for a template, Select that template, in textInput2 search for a user they want to send to, in textInput1 continue to edit/add on to the template they have selected, then click the send button and send their email.
The only thing I would change is I would build the main portion of the email process into a Power Automate Flow that is called that way you can control which connectors a run-only user is operating under incase you want to use a company-wide email or something.