Unlike other metadata types EmailTemplate
metadata type does not support a wild character.
Also, it will need you to specify the folder name as well to retrieve the metadata.
It will be some manual work unless you script it using Bash or Node.js,
Here is how I would approach manually,
Step 1
Create a SOQL to get all the folder names of Type Email
sfdx force:data:soql:query --query "Select Id, Name, DeveloperName, Type, NamespacePrefix from Folder where DeveloperName!=null and Type='Email'"
Step 2
For every folder in step1, note down the developername
and use the below command.
sfdx force:mdapi:listmetadata -m EmailTemplate --folder=<developernameoffolder>
Step 3
From the output JSON
look for the field fullName
to construct a package.xml like below
<?xml version="1.0" encoding="UTF-8"?>
<Package xmlns="http://soap.sforce.com/2006/04/metadata">
<types>
<members>unfiled$public/MarketingProductInquiryResponse</members>
<members>unfiled$public/SalesNewCustomerEmail</members>
....
....
<name>EmailTemplate</name>
</types>
<version>53.0</version>
</Package>
Retrieve using package.xml using the mdapi
sfdx force:mdapi:retrieve -r ./mdapipkg -u <username> -k ./package.xml
Alternatively sfdx force:source:retrieve -m "EmailTemplate:<folderdevelopername>:<templatedevelopername>"
for n times.