What operations on a c++ 11 atomic variable are actually atomic?

I'm reading the cpp reference manual on std::atomic (https://en.cppreference.com/w/cpp/atomic/atomic) and I'm confused about the exact operations that are actually executed atomically.

The main operations I'm having trouble with:
operator=, operator+, and operator+=
I know that these operations are defined for the template I'm just unsure if and when they actually are atomic. I'd appreciate help understanding when an operation on such a variable is atomic and not.