How sin90degree equals 1?

Sin(Sine) is a cyclic curve.

This cyclic curve(wave) that repeats every 360 degrees ( or 2pi radian).

Its Amplitude ranges from (+)1 to (-)1

When learning about trigonometry(Trig.) you only learn the bit between 0 degrees and 90 degrees.

On extending this learning you will find it to be a cyclic curve.

The Word 'Sine'(Sin) is from Latin and means 'curve'.

The Word 'Cosine(Cos) means 'complimentary sine' . It is the same curve as Sine but shifted along the x-axis by 90 degrees.