Hi Benjamin, welcome to Discourse.
See Create and configure custom user fields
To add additional info you can create a simple theme component and add it to the main theme(s) on your site. You need only to insert in the Common/CSS tab something like this:
.d-modal.create-account .login-form table:before {
content: " EXTRA TEXT HERE EXTRA TEXT HERE EXTRA TEXT HERE EXTRA TEXT HERE EXTRA TEXT HERE EXTRA TEXT HERE ";
}
to have this result both on desktop and mobile
There is already something similar at the end of the form, but you can add more text if needed. Go to the /admin/customize/site_texts
page, search for By registering, you agree to the
to find the correct template and edit it or add more text.