const links = [
'https://youtu.be/fi5LDM9ppuc',
'https://www.youtube.com/embed/fi5LDM9ppuc',
];
const newLinks = links.map(n => `https://www.youtube.com/watch?v=${n.split('/').pop()}`);
String str1 = "...";
String str2 = "...";
String str3 = "...";
String myString = str1 + str2 + str3;