Control over responsive layout

I am reasonably proficient at HTML and CSS (and can cope with php a bit) and I’m looking at how to control the various changes that occur once the site in viewed on a mobile device (or a “thinly” re-sized browser). I am happy with how everything looks in a full size browser but things go a little south on a mobile.

Can someone please point me to some documentation that will show me how I can position and re-size certain elements on my site when in “mobile” mode.

I believe this is all to do with “responsive” layout. In short, how do I control it, where are the docs for this, do they exist? If not, which files in the theme do I look at? Will bringing them into a child theme and editing them work in the usual way?

TIA

Richard