Analyzing the force of the hinge is to analyze the force of each bar separately. Through the interaction of two bars, the direction of force can be expressed by the component forces along the bar (X) and the vertical bar (Y), so the direction of resultant force can be determined by the component forces in the X and Y directions (the direction of resultant force of fixed-end hinge is not constant, or not along the bar).

Supplement: In addition, there are forces on both fixed and unfixed supports. The fixed bearing is similar to the upper part, and the non-fixed bearing only has the force perpendicular to the sliding surface.

So if the hinge is fixed on a plane and connected to a rod, will its force be perpendicular to the rod? ...

Not necessarily/possibly

Imagine that if you put the rod perpendicular to the plane, you give it a force perpendicular to the plane with your hand, and this force is of course on the rod at this time. But at this time, you add a force of 5 Newton parallel to the left at the top of the rod (the point where the hand applies force) and a force of 10 Newton parallel to the right in the middle of the rod, and then the rod is still balanced, but do you say that the resultant force on the hinge is still along the rod? Of course, it is no longer along the pole, and the direction is determined by the combined force of the downward force of the hand and the horizontal force of 5 N to the left.