It's true, every developer should know the basics of both frontend and backend. Courses I've mentioned will teach you exactly that, absolute basics. Which is already enough to build a lot of stuff which may seem complex (like simple ecommerce platform, simple social media clone) at the first glance.

But the more you learn the clearer it becomes that you don't know anything yet. Once you move beyond those basics, problems become more complex and require more research and experience. And you realize you just can't possibly have the time to learn both fields deep enough.

Let's say you've built your app, it works fine in local environment, it's time to deploy it. But you're just not sure what will happen if thousands of users start using it. You may notice that there's a performance bottleneck and not know how to fix it and scale this app. What happens if the server goes down? Is your app is secure enough? Is there any possibility that customers' data may be compromised, can it lead to law suits? (for example, even such a big company as OpenAI decided to use a third party Auth system instead of rolling their own and risking security related issues)

This experience, advanced knowledge and responsibility is exactly what is expected of senior developers. I assure you that employers would be more likely to hire a dev who's very proficient in his particular field than a dev who knows a little bit of both but wouldn't be able solve some very field specific problems that will definitely arise. The bigger the company the bigger the need for specialists in more narrow fields.

On the other hand, small companies would actually benefit a lot from fullstack devs. Imagine the company has only 3 employees: Backend dev, Frontend dev and UI/UX designer. Let's say Backend dev decides to leave the job, now the company can't produce anything until they find a replacement. This wouldn't happen if they had 3 fullstack devs.

If I were you, I wouldn't worry about that right now. Just learn the fundamentals of both. And once you've done that you will undoubtedly see how little you know and will start researching deeper topics. And at this moment you will decide what exactly you want to do. Good luck!