Where in Bitrix can I see the values ​​of the fields that can be set, for example, for a mail template?

Question

1C-Bitrix

When editing a mail template, you can specify the following fields:

Where can I see their values?

Answer 1

These are variables. And what will be transmitted in them will be.
Send yourself an email and see

Answer 2

The data gets into the mail template from the CEvent::Send or CEvent::SendImmediate method. For example:

$arMailData = array(
    "NAME" => $arFields["NAME"],
    "LOGIN" => $arFields["LOGIN"],
    "PASSWORD" => $arFields["PASSWORD"]
);
CEvent::Send("NEW_PARTNER", "s1", $arMailData, "Y", false, false, "ru");

The third argument is a data set, respectively #NAME#, #LOGIN# and #PASSWORD# will be available in the email template on the "NEW_PARTNER" event.
If you need to find out how the data set for a letter is formed, you need to find the CEvent::Send call for the corresponding mail event. Dispatch methods are usually called within components or module classes.