Does the prohibition sign placed under the built-up area sign indicate that the prohibition applies to the entire built-up area with the exception of the section of the road on which it has been replaced or cancelled?

In the video you are entering a town on a road through fields; on the right there is sign D-42 "built-up area", and under it sign B-33 "speed limit" of 40 km/h. A prohibition sign placed under sign D-42 means that the prohibition applies in the whole built-up area, except for the section of road on which it has been changed or cancelled by another sign. The answer is: yes.

The Regulation on road signs and signals states that a prohibition sign placed under sign D-42 means that the prohibition applies up to sign D-43 or E-18a, except for the section of road on which it has been changed or cancelled by another sign - and signs D-43 and E-18a mark exactly the end of the built-up area or of the town. This is an exception to the usual rule according to which a speed limit from sign B-33 ends at the nearest junction, if it has not been cancelled earlier. Here junctions change nothing: through the whole town you drive at a maximum of 40 km/h, unless another sign B-33 introduces a different speed or sign B-34 or B-42 cancels the limit - and only on that section. The statutory 50 km/h for a built-up area is replaced here by the lower limit from the sign.

Legal basis: § 15 para. 4, § 27 para. 1 and § 58 para. 3 of the Regulation on road signs and signals; Art. 20 para. 1 of the Road Traffic Act (Prawo o ruchu drogowym)