A function for determining an ACF field's sub field key based on a field name. For more info see https://philkurth.com.au/tips/get-an-acf-sub-field-key-by-field-name/

get-acf-sub-field-key-by-field-name-function.php

<?php

/**
 * Locates an ACF sub-field by field name and returns the sub-field's key.
 *
 * This is particularly useful if you need to construct a data array for programmatic field
 * update where a complex field is in use (e.g; repeater, group, flexi).
 *
 * @param string $sub_field_name The sub field name we need a key for.
 * @param array $field The ACF field array.
 *
 * @return string The sub-field's field key or an empty string where the sub-field is not found
 */
function get_acf_sub_field_key_by_field_name( $sub_field_name, array $field ) {
	$sub_fields = acf_get_fields( $field );

	foreach ( $sub_fields as $sub_field ) {
		if ( $sub_field['name'] === $sub_field_name ) {
			return $sub_field['key'];
		}
	}

	return '';
}