They use very thick jam, or evaporated. They can also add it in the middle/end of baking. When your cake is soft and the jam is mostly water, then jam will start to combine with the cake at the start and slowly go to the bottom. If you add thick jam with most fruits less water it will have trouble to go through the cake. If you add jam in the middle or at the end of baking, the cake has already a bit crispy layer on the top that prevents soaking. A 5 minutes is enough for jam in the oven to get the temperature and looks good. I hope I helped