How is magma formed?

Magma is extremely hot liquid or semi-liquid rock. Magma is the same thing as lava, except that lava is what we call magma when it reaches the earth's surface. The liquid rock that comprises most of our planet's mantle layer is known as magma.
The minerals that form magma are the same as the minerals in other rocks in the earth's crust. The reason much of the rock in the mantle layer of the earth is molten, or liquid, is because of the high temperatures and pressures formed by the weight of the earth's crust.
Magma can be formed by either wet or dry melting. Dry melting occurs when a rock that doesn't have carbon dioxide or water present reaches its melting point. Wet melting occurs not just at the mineral's melting point but at a variety of temperatures, because the presence of carbon dioxide or water changes the melting point of the substance.
There are three types of magma, and each is formed from different conditions. Basaltic magma forms through dry partial melting of the earth's mantle, which lies directly under the crust. Convection causes the hot mantle material to rise closer to the crust, which warms the crust. This increased crust temperature then increases the temperature gradient in an area enough to cause partial melting of some mantle material.
When basaltic magma rises higher in the mantle, it can increase temperatures in an area of the earth's crust. Partial wet melting of the continental crust produces another kind of magma: rhyolitic magma.
When continental plates pull away from each other under the ocean floor, pressure increases and water rushes into the mantle layer. Water lowers the melting point of the minerals, so partial wet melting occurs. This produces basaltic magma. If this basaltic magma comes in contact with continental crust containing dense dioxide silicon and mixes with that melted material, it forms the third type of magma: andesitic magma.
https://opentextbc.ca/geology/chapter/3-2-magma-and-magma-formation/