There is any loss if you learn anything new. Simply you will just add another skill on your portfolio.
If you want to use bootstrap then first you must need some basic knowledge about html css. Then you will be able to use bootstrap properly.
Get 200+ Hours Free Courses.
Also big websites prefer custom coded website from scratch. Because it gives you complete freedom to customize the website.
I will recommend you to learn Basic HTML, CSS & JavaScript first. Then try bootstrap. You will love it. Here you will find some beginner friendly tutorials for both bootstrap and custom coding.
