Is it necessary to study Bootstrap as a web designer or not?

It isn't necessary but it is useful. Bootstrap makes a lot of things look good by default (especially forms), gives you handy responsive layout tools by default (like containers and grids). You could build all of this yourself, but that's a lot of work. And their framework is battle tested in all sorts of scenarios you might not have thought of yet. You can extend and customize Bootstrap so it looks like your brand rather than generic.

I've used Bootstrap in personal projects. At work we have our own Design System/CSS framework that's not based on Bootstrap. But there are companies that use their own flavor of Bootstrap. There are other CSS frameworks too like Material Design from Google, Zurb Foundation, Semantic Design. My advice is less about using Bootstrap specifically, and more about the benefits of using a CSS framework, and pragmatically that you'll probably have to work within one at some point.

Building on Brendan's answer, I think it depends really on what sort of design you want to do. Some designers build prototypes entirely using Adobe or some other software package and leave the building of the site to developers. Other designers code themselves as well and prototype in the browser. I would recommend it, but it depends on the nature of the work you want to do. But it can never hurt to have an additional skill in your toolbox.