School is hard. Let the internet help you.
You know those classes where you start confused and two hours later, you leave the lecture even more bewildered and confused than when you entered? Me too. Over the years, I've found several resources to help me with my math and science courses (especially math oof differential equations was rough) and thought it would be selfish to keep them all to myself. So, here ya go.
This amazing non-profit needs no introduction - whether it's trigonometry or introduction to Fourier series, Kahn Academy has it all. It's 100% free to use and makes learning accessible to everyone. I've used Kahn Academy myself, especially for multivariable calculus, linear algebra, and differential equations. Although some of the videos aren't necessarily robust enough to cover all college-level science material, I've found that it's enough to get your feet wet and make those lecture notes slightly more understandable.
three blue one brown
Created by Stanford math graduate Grant Sanderson, Three Blue One Brown is your gateway into higher level mathematics. His series "Essence of Calculus" and "Essence of Linear Algebra" are perfect to supplement any STEM student taking undergraduate mathematics course who may be having a tougher time visualizing the concepts. He also teaches on Kahn Academy on multivariable calculus series. All of his content is available for free on YouTube.
feynman lectures on caltech
Ever heard of the Feynman technique? Richard Feynman, whom the technique was named for, was a Nobel Prize winning theoretical physicist and lecturer at Cal Tech. His lectures, famed for their easily digestible yet still robust nature, are available for free on Cal Tech's website. From Newton's Laws to Quantum Mechanics, Feynman has lectures for everyone regardless of the level of their physics education.
Here are also a few honorable mentions for learning how to code as well. I can't attest to their robustness, but have heard only good things about the following and will be updating my thoughts on the sites after my own use:
CodeAcademy - primarily for learning Python
Learn-cpp.org - primarily for learning C++
MIT Open Courseware - for both Python, C++, and other languages.