How to work with hybrid webapp (MPA and SPA combined)
Their is a lot in this question, as a starting point you can define multiple entry points in webpack.
https://webpack.js.org/concepts/entry-points/
If you want to mix data loading between FE and BE then you really need to write an isomorphic JS application and use Node as your BE, otherwise you’ll end up writing everything twice in different languages and having once come across a project like that, trust me you really want to avoid that.
The other bit of this question on shared resources is best answered by WebPack’s bundle splitting which is made for what is being asked here
Not sure if I totally understand the question, but single-spa (yes it's redundant) is a tool that can be used to combine multiple apps (even if they are different frameworks) into one single page application. Link to the docs: https://single-spa.js.org/docs/getting-started-overview