What is inside of a cave?

Answer 1

Inside a cave is, hopefully, air so you can study the cave.

But many are filled with water, and most have some liquid since this forms the majority of limestone and other soft rock caves. Lava caves are different but also may be formed by ground water.

Inside caves, you may find rock formations: stalactites from the ceiling and stalagmites from the floor, formed over thousands of years by minerals in dripping water. In shallow caves, you may find animals such as bats, bears, snakes, and rodents. Sometimes caves become the home for flocks of birds.

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That second paragraph is rather vague.

Many cave passages are indeed filled with water, and caves in limestone, rock-salt and gypsum are formed by water dissolving the rock's primary mineral (the rock's chemistry is important, its hardness, less so), but over time caves can lose their formative streams completely. A few limestone caves form by dissolution by highly-acidic, mineral-rich water circulating from below; but these are very unusual. Most use rain and snow-melt water from the land surface.

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Lava caves differ completely: they form in basalt lava flows by molten rock draining from below the solidified surface - nothing to do with ground water.

Answer 2

The interior of a cave can vary depending on the type of cave. Common features found inside a cave include rock formations such as stalactites and stalagmites, underground rivers or lakes, and various types of minerals. Some caves also contain animals adapted to living in the dark environment.