Question

why are there triangles in the structure are the triangles congruent? when are two triangles are congruent?

Answer 1

congruent simply means "equal"
if 2 triangles are congruent, then their size and angles are the same (but their orientation can be different)



so for the above triangles... (first imagine my drawing skills were better)
if you were to print that image off and cut them out
the triangles would be congruent if you could place one perfectly on top of the other and they matched

Answer 2

also, welcome to OS @kienzrose ;D