Answer by Japanese teacher
A: 明日(あした)ちょっと病院(びょういん)へ行(い)きます。Ashita chotto byooin e ikimasu. (I’m going to the hospital tomorrow.)
B: そうですか。お大事(だいじ)に。Soo desu ka. Odaijini. (I see. Take care.)
It would be weird if you say naruhodo here because you don’t have to agree or accept the fact that A is going to the hospital. Soodesuka is more appropriate here.
A: こうすれば簡単(かんたん)ですよ。Koosureba kantan desuyo. (It’s easy if you do it like this.)
B: なるほど / そうですか。Naruhodo / Soodesuka. (I see.)
For example 2, you say naruhodo because you agree and accept what A has said that it is indeed easy if you follow A’s instructions. You can also say soodesuka here because you realized something that you did not realize before.
I hope this explains how to use naruhodo and soodesuka.