The sentence needs a verb between pwy and wedi ennill. A literal translation might be 'who (is it who is) after winning?' - sy here is a special form of ydy which provides the 'is it who is' element.
- Pwy sy'n ennill? - who (is it who) is winning?
- Ble mae'r bobl sy'n byw yn y Rhyl - Where are the people who are living in Rhyl?
Apparently sy is introduced in Emphasise1, 12 chapters later than I'm currently up to
A brief explanation has now been added in the notes for this section, 'Describe 1'.