I'm using react and I wrote my own small useObservable hook that lets components subscribe to the value of an rxjs observable
https://github.com/hzrd149/applesauce/blob/master/packages/react/src/hooks/use-observable.ts
I don't know if svelte has native support for observable, but Im sure you could write a really simple helper method that would allow components to use them. ( want to build a applesauce-svelte package 😉 )
Applesauce is broken into a few parts but the core of it is a large collection of helper methods https://hzrd149.github.io/applesauce/typedoc/modules/applesauce_core.Helpers.html and the in-memory EventStore https://hzrd149.github.io/applesauce/overview/events.html
The EventStore ( and QueryStore ) are designed to store all the events the UI layer needs and they expose a bunch of different observable methods that let components subscribe to events ( timelines, lists, profiles, zaps, replies, etc )