I think that left line is padding on body in smaller screen media query. It gives your content a little room on the sides but also pushes the header with it.
Your min-width and max-width on container seem to be working against each other so you end up with 100% wide page no matter what. I’m assuming you want a header all the way across but also want to control the width of the other content at different screen sizes. Separating header from container would help there - then style the container, not the body.
The input elements can have margin just like any other element so you could use that to put some space between those form elements.
The textbox is a bit larger than the other inputs because it comes with some padding supplied by the browser.
box-sizing:border-box will make these all size similarly.
The video doesn’t size responsively at small screen. You might want to google that - there are some ways to control that but it is hard to explain.