Page number in Footer problem

Pages v13.2 headers and footers cannot accept document objects, so your belief that you inserted a text box in the footer would be false. It floats above it.

All one has to do is click on one of the three footer segments and insert the page number. It will be right-aligned and there is no point to changing its paragraph style to body text. The footer isn't very tall on purpose because body text footnotes must fit between the footer and the body text. If you manage to make the footer too large, the footnotes will cause the body text to shrink upward.

I suggest you use Pages as it was designed and not pay the price later when your changes create document or export format compromises. Just because MS Word can optionally insert a 1-inch tall header and footer in its documents does not mean you should attempt to modify Pages documents in the same manner.

All that video describes is using a text box grouped with shape elements and then moved over the header and then positioned behind the header using the Arrange panel to move the object to Section/Layout master. That is really unnecessary.

Presumably, you imported a Word (or possibly a Pages '09) document to get your headerless/footerless document view. All you need to do is visit the Documents panel and select the Header/Footer check boxes and you will then have an unsegmented header or footer. You can then click on that footer, a floating Insert Page number appears and you click that to insert the page number.

That page number will be left aligned by default, and the Format panel controls for left, center, or right aligned can be used to place the page number in any of those positions. No need to bother with any paragraph style.