I want to study plus code data structure from basic, which online class ( free) or website is best for it?

You can find some of that on SoloLearn itself 🙃

I would look at the top results from a web search.

I don't know if it is the best but you can use geeksforgeeks https://www.geeksforgeeks.org/data-structures/ I prefer using it coz it's really wide

Sololearn 😉