You probably used Khan Academy in school and forgot about it, but for foundational subjects it remains one of the best resources on the internet, and it is completely free.
The mastery system is what makes it special. You practice a concept until you demonstrate mastery, then move on. That structure prevents the gaps that sink learners later.
It shines at math, science, and the fundamentals that underpin technical careers. Before you buy an expensive prep course, check if Khan Academy covers the same ground for free.
Pair it with a goal, and it becomes a serious learning engine. Many people build their entire foundation here and then spend money only on advanced, specialized material. Start free, master the basics, spend later.