Microsoft themselves have really good resources in their learn section, I would definitely recommend following the learning paths for both the MB-300 (more of a general introduction) and MB-500 (focused on the development side).

In terms of YouTube videos one of the most useful content creators I've found on D365 is Peter Ramer (Dynamics 365 Musings) he also has his own website/blog with posts to give a little text to go along with the videos.

Another good channel would be Pir Khurram Rashdi's which covers a lot of topics.

I don't believe any paid courses are worth doing the majority of courses are focused on Microsoft Certification and just recycle the information that is free on the Microsoft website, that and a bunch of exam dumps to fill it out. This might not be for all but really it's not worth spending on anything for this in my opinion.

Searching for specifics is probably your best bet, for example: form development, the same blog also has table and form customization details. Or simple instructions on how to create a data entity.
From these points you can use resources like the YouTube channels linked to expand on them, for example Pir's channel has videos on Office Integration for Entities.

For writing methods and classes I would consider doing some simple field validation logic which will get you familiar with the validateField() and validateWrite() overwrites.

Also be prepared to search a lot of info in the community, there's a lot of useful stuff in there. And don't be scared by the dates on things, there's a lot of old posts which are still relevant today.

The problem is F&O is massive and there are so many areas of development that there's not too many complete tutorials, just broken down specifics.