From sucking up nutrients and water from the soil (through their roots), and CO2 from the air (through small pores under their leaves, called stomata).

This is 7th grade biology.

The sunlight is only a catalyst in the chemical reaction called photosynthesis.

The reason sunlight is a catalyst is exactly for the same reason as I described earlier - it makes the electrons more excited, and causes them to move into bigger orbits away from the nucleus of the atoms - which makes it easier for another atom to break existing bonds, capture that electron, and form a new chemical bond).

For more information, please check wikipedia and basic biology, physics and chemistry textbooks.