Look at this diagram which shows four right angles placed next to each other at a point.
Here is 1 right angle, which is 90°.
Here are 2 right angles put together, giving an angle of 180°.
2 × 90° = 180°
Here are 3 right angles put together, giving an angle of 270°.
3 × 90° = 270°
Here are 4 right angles put together, giving an angle of 360°.
4 × 90° = 360°