As I add posts about this site, I'll presume you're already familiar with the basics of Python, Django and Wagtail.
In case you're starting out, I would learn those in that order.
It's hard to beat Real Python for everything from beginner to advanced topics ranging from coding fundamentals to web development, machine learning and more.
As for Python, there are a ton of resources out there for learning all there is for this Python based web platform.
The Django Book is the full documentation from the Django Project itself and kept up to date. There's not much about Django you can't learn from here.
Mozilla's Developer's site also has a very comprehensive site including free coursework for beginners:
Django Web Framework (Python) - Learn web development | MDN
Django is an extremely popular and fully featured server-side web framework, written in Python. This module shows you why Django is one of the most popular web server frameworks, how to set up a development environment, and how to start using it to create
There are much fewer resources for Wagtail, but don't let that put you off. The benefits of using Wagtail far outweigh the lack of material out there. Once you see Wagtail's streamfields in action, you'll never look at Django again (almost).
Learn Wagtail has an excellent set of free and paid-for (for a very low fee) instructional videos that'll take you from beginnings to early-advanced topics. I would say start here. You can maybe skim the Headless CMS and PET videos if you're pressed for time.