The points in a coordinate plane are reflected in the line y = x. in general, every point (x, y) is mapped onto what point?

We know that
A transformation which uses a line that acts like a mirror with an image reflected in the line,  is called a reflection. The line with acts like a mirror is called the line of reflection

in this problem
the line of reflection is
y=x

if (x,y) is reflected in the line y=x its image is the point (y,x)

therefore

the answer is
(y,x)