https://www.codechef.com/LRNDSA03 here is this https://mirror.codeforces.com/blog/entry/55274 https://mirror.codeforces.com/blog/entry/111099 roadmap including some stl problems check out the ultimate topic list in codeforces catalog go to basics open them and you will find many problems includding stl.I know it is not much but try them , an advice from my little experience is when i learned stl i did not practice on it much just paractice a bit so you inderstand them well and then just do problems and you will realize you need it you check out cses sorting and searching they really need sets and maps and they are good practice for effective use.Hope it helps and good luck.(for anyone reading this comment please stop downvoting gray blogs and help them instead).