How are congruent triangles proportional by a scale factor if 1

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I believe that you meant to say, "of 1".

That's simple; by definition, two triangles are congruent if and only if they are identical--corresponding sides and angles are likewise congruent. Two triangles are similar if the corresponding angles are congruent; then the corresponding sides are proportional by some factor. Since all congruent triangles are similar, their scale factor must, by necessity, be 1, since they're exact copies of each other.