The emphasis in a Welsh word is always on the last but one syllable.
eg 'mynydd' = (a mountain) the emphasis is on 'myn' but for the plural; mynyddoedd (mountains) the emphasis changes to 'ydd' because this is now the last but one syllable.
Exceptions to this are usually indicated with an accent; eg 'casáu' = (to hate) the emphasis is on the 'au' because of the accent.
There is no difference in this whatever the dialect.
In the video the 'y' sound is sometimes subtly different between North and South but the emphasis is still the same.
The diaresis (the two little dots) indicates that the vowel it is over is a single vowel not part of a digraph with the following vowel - an example being copïo which is pronounced (as near as I can illustrate it using English) copy-o not cop-yo
There are various places you can find an explanation - the one I tend to check is https://en.wikipedia.org/wiki/Welsh_orthography#Diacritics