How to create a reusable component in another applications in reactjs?

Probably npm private package is what you are looking for. Then, you can just install your package by npm install and use components from it everywhere you want. However, this is paid feature of npm.

The alternative is an npm proxy registry. For example Verdaccio allows something like that. Similiar solution to the previous one, but you have to configure the registry yourself and use it in all your projects instead of default npm registry.

You can also just publish a public npm package if it's ok for you that it'll be available for everyone. It's probably the easiest solution for starters.

Alternatively, what we used in our project, were Git submodules. You place your shared components into a separate Git repository. Then, you link that as a submodule in you main project and you can use your components just like they were a part of your project. If you use Git, it might be the easiest option for you.

And finally, if you treat you projects as playground, you always can use copy-paste approach :) I don't recommend it in any commercial projects as maintaining the code in many places is pain in the neck.