You set the wording of your site's Welcome Email, both the subject line and the body, at Admin Tool → Communications pill → Welcome Email. Super admin access is required.
Note: what you save here is the Welcome Email for all users, including users created by the datafeed process.
The body of the email is edited in the text box. Clicking into the Email Subject Line field opens an input editor for the subject, and Save commits both.
If your site has multilanguage enabled, the subject line can be set for each Welcome Email language variation you have enabled. See the Multi Language Tool Overview for how language variations work.