Hello,
I’d avoid the word “code” unless you’re referring to HTML/CSS/HubL to avoid confusion. Most email templates are made in the WYSIWYG editor that should be in design tools. It uses a 12 col. layout and you can create groups of modules and add some CSS and such to modules/groups as needed. So you won’t “code” the email as a whole but tweak the modules/groups themselves.
I think this article should get to the heart of your issue.
Also, here’s the Knowledge Base topic on Design Manager that might be helpful as you work.
Hope that helped!