To create a blog post in Builder and then have it show up as a post in your Shopify blog, you need to create a new page in Builder and then embed it in Shopify.
To do that, first, head to the page in Builder that you want to use as a blog post. Make sure the page is published, and then click the three dots in the upper right hand corner. Click “get code,” and then select “wordpress” as the code type. Click to copy the embed code, and then head to Shopify and create a new blog post. When you create the blog post click the button to show the html view, then paste in the embed code you got from Builder. You can then publish the blog post on Shopify, and the builder content should show up on your Shopify blog.
Here is a short video that goes through the process: https://www.loom.com/share/c51c74247f6d4c5e882831b06ed1da92