Khan Academy: https://www.khanacademy.org/math
Paul's Online Notes: https://tutorial.math.lamar.edu/
3 Blue 1 Brown: https://www.youtube.com/@3blue1brown
Favorite online textbooks:
Discrete Mathematics: An Open Invitation by Dr. Oscar Levin: https://discrete.openmathbooks.org/dmoi3.html
Counting Rocks! An Introduction to Combinatorics: https://arxiv.org/abs/2108.04902
(For graduate students) Notes on Class Field Theory by Dr. Kiran Kedlaya: https://kskedlaya.org/cft/frontmatter-1.html