What training tools do you recommend?

I've been discovering some independently or through recommendations from others, such as the "Codeforces Custom Contests" extension and "ThemeCP", which I find very useful. Do you have any others?

Do you recommend training for individual problems and then participating in an official contest, or do they recommend doing a virtual or customized contest for training?

how can I add new friends in Codeforces?

I try to add some classmates to my school, but I don't get a button to add them anywhere. Is being able to add friends dependent on contribution points or rating?

I would like to know if having a negative contribution does not allow me to create groups, since I want to create a private group for the people in my school who are interested in learning to program and I can send them the problems from here and it is easier for me to have to correct what they send.