The actual structure of the alternator in a power plant is much more complex than this, but it essentially consists of two parts: a magnet and a coil. Magnets use electromagnets and often more than one pair of magnetic poles. The coil also has more than one turn, but multiple turns.
Some generators also install the coil in the outer housing to keep it fixed, and install the magnet on the rotating shaft. When the magnet rotates, the magnetic field lines also rotate.
As the movement is relative, the fixed coil does not need to pass through copper rings and brushes to deliver current. This eliminates the risk of sparks when the brush and copper ring are not in good contact.
High-power generators all adopt this rotating magnet structure.