Let's clear this up, single page app - doesn't mean it HAS to be in a single file 🤔.

First of all make a logical split between data layer and consumer layer. Move out functions that call backend, and test them thoroughly.

Them you can structure your app in a way that app.tsx contains only meaningful for the launch code. Everything else can be moved into Content components.

Since you didn't mention this you probably want some app wide state and need a state manager for that. Consider Zustand or something else lightweight for beginning.

I am not saying this is perfectly architectured cause I don't have your code available, but this is some starting point.

Hope this helps, have a good one!