How can I preview my online store app extension without uploading code?

I want to develop an app which will directly interact with the end users.

This will app will be inside online store.

I went through the documentation and found that it can be done with the help of Extensions for online store.

After creating Extension → Online store → Theme App Extensions I am getting a message of “Push code to preview this extension”

Is there a way in which I can preview my app without uploading the code?

Is there any tutorial which I can follow to create an extension?