How can an app be truly universal?

Firstly, I’m sorry if this question is dumb/in the wrong category, but I couldn’t find a more suitable category.

I’m fairly new to development and only heard about Expo/React Native through a workshop a while ago.

I’m struggling to understand the concept of an app being universal across Android/Mobile/Web. Do mobile apps not differ quite significantly in the way they work, with the user interface for example?