I would like to answer your question even it is very subjective.
- How SharePoint Saves Modern Pages?
So SharePoint online use page blob to store and save the pages.
- How it renders Modern Pages?
Modern page work and render in similar fashion as react .
- Mounting phase- constructor,render, componentDidMount,
- Updating phase- - ShouldComponenUpdate,Rendor,ComponentDidUpdate
- Unmounting phase - ComponentWillUnmount.
for more details refer the below screen
3- Does Ghosted & Un Ghosted concepts applies to Modern Pages?
4- How Modern Pages saves web part details & their properties?
State and Props
5- How routing works in modern pages, as it behaves like SPA?
Most of pages uses Hyperlink concept. You can explore this part by yourself.
Please mark this as answer if you are satisfied