Developer Mode - Learning Resources?

I'd like to be able to offer template customization, but I can't find a good place to learn!

How did you learn to work with developer mode, what tools do you use, and where would you suggest I go to learn how to make a template from scratch?

Thank you!

More details: 

I'm familiar with HTML/CSS/JS but JSON-T, .region, and the other configuration files are new to me. I've been working with VSCode trying to make changes to a trial-version developer-mode site, but after a number of hours I've only been able to follow the super basic tutorial and add some static text to the main page with git push.

I'd like to learn how to work with .region files and how to use different .region files on different pages. With that information, I hope I can build a fully-custom website with Squarespace's CMS on top.

I've googled it in different ways, but the results are always flooded with Squarespace's WYSIWYG editor tutorials and the few resources that match my search terms are expensive courses that, I assume, don't have detailed instructions I'm looking for.