Hi there @Dalveer_pro,

First, welcome to the Community! I hope you find everything you need here regarding information on product blocks and email templates.

In our article, How to Insert a Product Block, we mention that Product blocks do not support any custom HTML. If you want to custom code your product block, you'll need to use an HTML template. So in this case I would point you to our documentation regarding importing a custom HTML template. From there, you can add custom code into your templates and product blocks that you would not be able to do with our built-in editor. 
If you run into issues regarding CSS and HTML, our troubleshooting article provides some solutions for common problems.  

I hope this could help!
Alex