There are good public resources but they are expensive. One of the best resources for learning in depth quantitative techniques that is actually legit as opposed to Youtubers telling you how to squint to see patterns in a bar chart is the ARPM, which provides both a week long course and a year long course:

https://www.arpm.co/

It is a very math heavy course, highly technical and requires a great deal of existing math background. But the truth is that you will not be able to do much quantitative finance without that background anyways, so if what you want is a way to gain that math background then just crack open a textbook and get working.