The lowercase w on a laughing kaomoji comes from 'warai' (笑い), Japanese for laughter — the JP equivalent of 'lol'. Stack it, wwww, and the laugh gets bigger.

A row of wwwww looks like blades of grass, so Japanese slang for 'that's hilarious' became 草 (kusa, 'grass'). Kaomoji laughter literally grew a meme.

In (≧▽≦) the ≧ and ≦ are eyes scrunched shut from laughing hard. The wider the ▽ mouth, the louder the laugh.

Sound-effect kana like ﾌﾟｯ or ﾌﾞﾌｫ behind a face are the sound of a sudden burst of laughter — the kaomoji version of a spit-take.

Sometimes the laugh is spelled out: (笑) literally means '(laugh)'. It's the ancestor of the modern w — a tiny stage direction for your own joke.