Using a stand mixer or hand whisk, beat the chilled heavy cream until stiff peaks form, which takes about 3-4 minutes. Look for the cream to hold its shape firmly and stand upright when whisked. Transfer to a separate bowl and set aside.
In a large mixing bowl, combine the sweetened condensed milk and vanilla extract. Stir until smooth and well incorporated, observing a glossy, thickened mixture that’s ready for folding.
Gently fold the whipped cream into the condensed milk mixture using a rubber spatula, turning from the bottom up until fully blended. Maintain as much airiness as possible, and notice the mixture becoming light and creamy.
Divide the mixture evenly into three portions. In two separate bowls, stir in the blueberry jam into one portion and the strawberry jam into another, creating vibrant, marbled color swirls. Leave the third portion plain for the white base.
Layer the three colored mixtures into a freezer-safe loaf pan, starting with the white, then placing dollops of blue and red mixtures on top. Use a skewer or toothpick to gently swirl the layers together, creating a marbled, flag-like pattern with visible streaks of each color.
Cover the pan tightly with plastic wrap or a lid and freeze for at least 6 hours or until completely firm. The final ice cream should be smooth, creamy, and easy to scoop, with distinct colorful swirls visible throughout.
