Go to Site > Site Settings > Notification Status. You can find an option 'Enable Welcome Emails'. By default, this option is turned on.
If this option is enabled, the System Emails generate when a user signs up to your site of a student enrolls in a course will automatically sent.
If you turn this option OFF, no welcome emails are sent. In that case you need to use the email automations for sending Welcome emails.
If you wish to set up automation for course enrolling, you can turn this option OFF. Otherwise they will get an email from the welcome emails and from the course automations as well.
If we click the question mark near the option, you can see what it says about this feature "if disabled system generated welcome emails for courses and site sign-ups will not be automatically sent, we assume you will be using automations to handle this"
See the video: