On an ad in the train today I saw 夢を叶える夏にする and I’m confused on what the に means when paired with する like that.
This explanation I found doesn’t really make sense because there’s no na-adjective in the sentence.
https://jlptsensei.com/learn-japanese-grammar/な-adjective-にする-ni-suru-meaning/
It means to make. It doesn’t have to be a na adjective and can be used for regular nouns.
It can indeed be used with any noun, and this linked explanation might make more sense to you than the one from jlptsensei:
Something like “I’ll make this into a summer that fulfills dreams.” I guess. I don’t know the context.
Anyway, this video should help:
5:03 is where it talks about になる, but にする is related to it in that, in the case of にする, the transformation is caused by the subject of the sentence (I turned her into a cat. 私が彼女を猫にした。), whereas, in the case of になる, the transformation happens to the subject of the sentence (She became a cat. 彼女が猫になった。). But it’s basically the same thing with regard to に.
I suggest watching the whole video even if you already know the material. Also, turn on the captioning. It seems to take awhile for most people to be able to understand Cure Dolly without them. She knew that and made her own captions for every video.