Two options - your air bottle has a regulator on it that drops the pressure down to either 7-800psi (called high pressure), or around 400psi (called low pressure). Some markers need low pressure, but most need to have high pressure.
Once the air reaches your gun, most high end markers have two regs on the gun itself: a high pressure regulator drops the pressure to around 150-250psi, with this air being used to actually propel the paintball (this reg is usually the foregrip). A second, low pressure, reg drops the pressure to about 75-100psi, with this air being used to move the internals of the gun (eg the ram and bolt) back and forth. This is usually the little sticky out reg under the gun's barrel.
Some guns use just a single reg on the gun, with other systems used to cycle the compnents (eg the PMR and Invert mini), and mechanical guns just use the output pressure of a bottle to both fire and cycle the gun....