They often dive to the bottom and spend many minutes under water by remaining motionless and therefore reduce the amount of oxygen they may need, but eventually they will have to come up to the surface for air. Like all amphibians, bullfrogs can breathe through their skin. However, even though there are hundreds of species of lungless salamanders that live on land, there is only one known species of lungless frog, and it lives in very moist, oxygen rich surroundings of cold streams. Frogs do not have the elongated bodies that salamanders have, so they have reduced amounts of body surface area compared to salamanders. That makes it harder for frogs to rely solely on breathing fhrough the skin. Since the bullfrog is also one of the largest frogs in the world, it means it has even less body surface to breathe with, since the larger an object, the smaller the amount of surface area it has proportionally. All these factors, plus the fact that bullfrogs often live in warm stagnant ponds, where there is even less oxygen than cold running streams, it means the bullfrog must rely heavily on its lungs to breathe. Therefore the bullfrog cannot stay underwater indefinitely and they must come back up to the surface to breathe.

Once they are adult frogs and no longer tadpoles, frogs lose their gills and instead use camouflage to remain almost entirely underwater whilst their nostrils breathe just above the surface.

In addition, frogs can absorb oxygen slowly through their skin, which allows them to remain underwater whilst hibernating.

Yes

They do absorb oxygen through the skin, so in this sense they breathe under water. However, they will need to breathe air too if they are to remain active, especially in warm weather.

During hibernation, though, they can stay on the bottom of a lake because they only need a little oxygen then. Also, there is usually more oxygen dissolved in cold water than in warm water.