You’ll want to check out the system_messages.welcome_user.text_body_template. You can edit it here:
/admin/customize/site_texts/system_messages.welcome_user.text_body_template?locale=en
(fyi, this is for the english version)
You can customize this and other site texts.
It’s also worth checking if you have the automation plugin installed. If you do, you can see the enabled automations with this url path: /admin/plugins/automation/automation