Once the project is created, you have to make sure you have added proper CORS origin to the Sanity Project. Otherwise, the API won't be accessible from your domain/website.
Head over to your project page and click API Tab.
Now you can see CORS origin settings.
By default, only http://localhost:3333
is enabled.
To work with Next.js, you have to add http://localhost:3000
For Astro Projects, add http://localhost:4321
Once you are ready, you must also add your production URL.
eg: https://myblogwebsite.com/
For each make sure "Allow credentials" is checked.
Once added, sanity will be accessible using the following URLs.
http://localhost:3000/studio
http://localhost:4321/studio
https://myblogwebsite.com/studio
Now, you can open your code editor to start customizing the content or the schema.