As a backend engineer who has experience with frontend I've learned it's two things
From an engineer's point of view, if they call themselves a full stack engineer they're usually a front end dev who have written a few apis in the past. No shame, but I've seen way too many people say they're backend without fully understanding what being a backend engineer really is in terms of scale, speed, and flexibility.
From q businesses point a view, a full stack engineer is a role they made up that means "we expect an expert in all areas but want to pay a lower rate".
Both areas need experts. I'd say I'm an expert in backend. I think it's impossible to be an expert in everything, and companies who want "full stacks" should expect jack of all trades master of none, and will attract literally any engineer, because what the hell is full stack.
I've probably pissed off a lot of fellow engineers, but I guess what I'm trying to say is be a master of your domain. Learn how frontend and backend work, but don't try to be an expert in everything. It's good to specialize. When you're asked if you are a full stack engineer it's perfectly fine to say "I have experience in the full stack, my expertise is on the backend, but I can do react when needed"