By RHS congruence condition if hypotenuse and one side of one right triangle are equal to hypotenuse and one side of other right triangle then both right triangles are congruent.
It is given that $$AB=BC$$ and they are hypotenuse of small triangles $$ADB$$ and $$CDB$$
So, In right triangles $$ADB$$ and $$CDB$$
$$AB=CB\cdots\cdots[\text{hypotenuse}]$$
And $$BD=BD=\text{common side}\cdots\cdots[\text{sides of right triangles}]$$
So, by $$RHS$$ condition
$$\triangle ADB\cong\triangle CDB$$