Where does the residue accumulate in the boiling point composition curve?
Let's start with the mixture $x_A$, which is made of 85% A + 15 % B. When heating this liquid, the first bubble vaporized appeared at a temperature just a little above $T'_A$, as given by the horizontal line. And the composition of the vapor is nearly pure $A$, with $1$% $B$. As a consequence, the liquid is loosing some $A$. its volume decreases, but as it does not loose $B$, its composition changes from $15$% $B$ to maybe $20$% $B$. As it has more B, it is necessary to heat the liquid to a higher temperature. The composition of the liquid is a point following the lower curve upwards to the right hand side of the graph. Meanwhile the composition of the vapor slightly changes and contains a little bit of $B$.
When the volume of the liquid has decreased by more than one half, the remaining liquid contains more $B$ than $A$. Its composition is $20$% $A$ and $80$% $B$. The boiling point of this mixture has increased : it is now about half way between the boiling temperatures of the pure $A$ and $B$ stuff. And the vapor gettin out of this liquid can be obtained in drawing a horizontal line from the point $0.8$ on the lower curve. This line cuts the upper line $y$ at a point $y_A$ = $85$%, and $y_B$ = $15$%. It is possible to continue heating the rest of the liquid. The amount of liquid is smaller and smaller, and the vapor has a composition following the upper $y$ curve.
Now, if you have a hot vapor containing the same proportion ($15$% B , $85$% A), and if you coli it down, the first drop of liquid happening at the beginning of the condensation has a composition corresponding the th horizontal line passing at $y_A, y_B$. So this liquid droplet contains $80$% $B$ and $20$% $A$.