How to find the block type of Super Table field that is inside a Matrix field
That's more complicated, because fields in a Matrix are scoped to the Matrix field, which is why you can't just go:

{% set field = craft.fields.getFieldByHandle('superTableFieldInMatrix') %}

You'll need to loop through the Matrix's fields to find the Super Table field, then carry on as normal.

{% set matrixField = craft.fields.getFieldByHandle('myMatrixField') %}

{% for field in matrixField.getBlockTypeFields() %}
    {% if field.handle == 'mySuperTableField' %}
        {% set blocktype = craft.superTable.getSuperTableBlocks(field.id)[0] %}