difference between standard’s atomic bool and atomic flag

std::atomic bool type not guranteed to be lock-free?

Correct. std::atomic may be implemented using locks.

then it’s not atomic or what?

std::atomic is atomic whether it has been implemented using locks, or without. std::atomic_flag is guaranteed to be implemented without using locks.

So what’s the difference b/w two

The primary difference besides the lock-free guarantee is:

std::atomic_flag does not provide load or store operations.

and when should I use which?

Usually, you will want to use std::atomic<bool> when you need an atomic boolean variable. std::atomic_flag is a low level structure that can be used to implement custom atomic structures.