Performing event on ClassList and Unable to get event.target from ClassList? [duplicate]

I have given btn class separately to all buttons. I want to get all buttons using .getElementsByClassName("btn") I know it return a ClassList. Next I want get a single button on which click event occur so that I wrote below code

let btn = document.getElementsByClassName("btn");
console.log(btn);        // successfully got all buttons in a class list

btn.addEventListener("click",(event)=>{
    
    let target_btn = event.target;
    target_btn.style.boxShadow = "inset 0 -1px 16px 1px black";
    target_btn.style.transition = "all 0.4s";

});

I want to get event occurred button (event.target) from ClassList so that I can add box-shadow & transition to that target button but unable to do.
In console, I also got Uncaught TypeError: btn.addEventListener is not a function