The sun and the moon appear to be about the same size, which is just a coincidence. In fact, the sun is a huge star, while the moon is just a satellite of the Earth. The diameter of the sun is 400 times that of the moon, making the size difference huge. But coincidentally, the distance from the sun to the Earth just happens to be 400 times the distance from the moon to the Earth. Therefore, from the Earth's perspective, the sun in the sky (commonly known as the "Sun Disc") appears to be about the same size as the moon ("Moon Disc").
The reason why I say "about the same" is that the orbit of the Earth around the sun and the orbit of the moon around the Earth are both elliptical. During one orbital period, the distance between the sun and the Earth and the distance between the moon and the Earth vary in length, which affects the size of the Sun Disc and the Moon Disc slightly. This is most clearly demonstrated during solar eclipses. During a total solar eclipse, the black moon completely blocks the sun, and the sky suddenly darkens, with stars appearing, indicating that the Moon Disc is slightly larger than the Sun Disc. During an annular solar eclipse, the black moon only blocks most of the sun's center, leaving a thin, narrow outer edge, and the sky remains bright, with no stars visible, indicating that the Moon Disc is slightly smaller than the Sun Disc.
Due to tidal friction between the Earth and the moon, the moon is gradually moving away from the Earth. Therefore, only in the past few hundred million years has the sun and the moon appeared to be the same size from the Earth's perspective, and we have been able to witness beautiful annular solar eclipses. Many hundreds of millions of years ago, the moon was closer to the Earth than it is now, and the Moon Disc was always larger than the Sun Disc, so there would be no annular solar eclipses. Many hundreds of millions of years from now, the moon will be farther from the Earth than it is now, and the Moon Disc will always be smaller than the Sun Disc, so there will be no total solar eclipses.