Things get unwieldy at a certain scale when you don't have something like Typescript regardless of whether you have a framework, but you could start by building up small bits of the frontend stack from scratch. I learned how to do a bunch of manual raw js stuff from scratch originally, then did some professional work with YUI at a huge scale which was probably as close to an event driven frontend system as you can get. Start by figuring out how to listen for url changes, make a network request library from scratch, store data somewhere.
Imo one of the beautiful things we now get along with frameworks is a host of independent build tools that make the feedback loop of testing changes nearly instant and loosely coupled from your code.