See the text in red box.
I have no problem while finding out the components of all 3 forces along OL but in the red box they are finding the components of 3 forces perpendicular to OL but i wonder what angles would be the 3 forces would be making the perpendicular to OL.
I want to know the angles otherwise i could have remembered it as by taking sin instead of cos.
There is no component of $P$ acting along $OL$ in the vertical direction.
The component of $P$ acting along $OM$ in the vertical direction is $P\sin C$ since if you extend $OL$ in the left direction, then the angle between the extended part of $OL$ and $OM$ is $C$. The angle between $OM$ and the upward perpendicular to $OL$ is $90-C$.
The angle between $ON$ and the downward perpendicular to $OL$ is $90-B$. Hence the angle between the extended part of $OL$ in the left direction and $ON$ is $B$. Therefore, the component of $P$ acting along $ON$ is $-P\sin B$. It is negative because it is acting down and the other forces are acting up.