How did we make the figure for A and B(I understand for AUB)

so, A U B contains max values of members from A or B
max value of x1 is 0.5
max value of x2 is 0.3
max value of x3 is 0.5
and, union is
A U B={(x1,0.5),(x2,0.3),(x3,0.5)

on a graph above (left), red line is representing set A, purple line is representing set B
a graph above (right) line is connecting only max values, so you take part of line which representing set A and is above a line representing set B, then take part of a line representing set B which is above a part of a line representing set A

here is one more example: