Learning by doing is a good thing, but if you dont grasp JS basics and only learn the framework (React) abstractions, you'll eventually have to re-learn things. I started learning jQuery at my first job, and eventually I've got up to speed with it and had a blast. But I didn't know shit about vanilla JS. So when I had to work with projects that didn't use jQuery it was almost as starting from 0 again. The point being, don't learn frameworks, learn languages. This will benefit you in multiple ways:

1. It will allow you to get a deeper understanding of the framework, by disecting and grasping it's core concepts and how the it operates under the hood (since its js based).
2. You'll have no problems when the framework eventually get replaced by a new one.
3. More options when starting new projects (e.g. is React/NextJs really the best option for this project I'm about to build)

The list could go on but you get the point