HTML Details and Summary and Flexbox CSS

I want to continue focusing on improving the use of flexbox , given that I am not yet completely comfortable with responsive layout techniques and that I need to refine my skills with such a concept. I also need to learn how to manipulate SVG.

Some HTML and HTML5 elements save you work in JS. The less JS you use, the better?

How to manipulate SVG with JS and CSS?

Is there a way to add an SVG file in the document with some Tag, through "src", without having to add all the code inside the document?

I got only background with CSS, with "background" and "url".