How the compiler knows that it needs to construct new instance of a class

Mystring x{"ABC"};

Mystring test="Hi there";

#include<iostream>
#include<cstring>

class Mystring
{
private:
    char * str;
    
public:
    
    Mystring(char * s)
    :str{nullptr}
    {
        str = new char[std::strlen(s)+1];
        std::strcpy(str,s);
        std::cout<<"constructor is called for: "<<this->str<<std::endl;
    }   
    


//overaloaded assignment operator
Mystring& operator=(const Mystring & rhs) 
    {
std::cout<<"copy assignment"<<std::endl;

      if(this == &rhs)
             return *this;   
             delete [ ] str;     
             str = new char[std::strlen(rhs.str)+1];
             std::strcpy(this->str,rhs.str);
       
        return *this;
    }
};

int main()
{
    Mystring x{"ABC"};

    Mystring y="Old value";
    
     y="new value";
    
    return 0;
}