I already have html/css website. Is it possible to insert bootstrap into it and use it only for some features , for example making this website responsive on all devices etc?
Not really.
Bootstrap sets CSS properties on many elements. Simply adding it will create a great many side effects.
Its effects are not limited to elements that you specify.
It also won't magically make things responsible. The responsive features in it require specific HTML structures which you won't have.
If you want to make an existing site responsive, you are best off writing your own media queries.
If you just want the responsive helper utilities and the grid, then you can import the grid only, so that bootstrap styles don't clash with you own existing styles.
So instead of importing bootstrap.css you just import bootstrap-grid.css.
If you use SCSS compiler you can do the same thing just with the .scss extension.
https://getbootstrap.com/docs/5.1/getting-started/contents/#css-files