When or how does || operator work with Return statement. Iam very confused

Linda

const friends = [
    {
        "fName": "mumu",
        "lastName": "baru",
        "langs": ["python", "r"]
    },
    {
        "fName": "isfaq",
        "lastName": "hussie",
        "langs": ["js", "python"],
    },
    {
        "fName": "mrX",
        "lastName": "das",
        "langs": ["c#", "java"]
    }
]

function friendLang(name, prop) {
    for (i = 0; i < friends.length; i++) {
        if (friends[i].fName === name) {
            return friends[i][prop] || "no such property"; \ Why does it work here
        }
    }
    return "no such friend";
}

console.log(friendLang("mumu", "address"));

this returns no such property Why does the || operator work in the above code base but not in the following -

const friends = ["mumu", "mrX", "isfaq"];

function nameCheck(name) {
    for (let i = 0; i < friends.length; i++) {
        if (friends[i] === name) {
            return `Yay ${name} exists!` || `Sorry, ${name} does not exist`;
        }
    }
}


console.log(nameCheck("mrY"));

this returns "undefined"

Where can I learn in depth about the usage of || operator in return scenarios and the different ways returns are put in place like the line return "no such friend"; in the first code base. I want to understand it completely.

I am sorry if the question or any part of this comes off as silly. Just getting started with Js