"Responsive Design" is a fad term to create more revenue for designers that suck at design but love to talk about rules.
EDIT: FOR THOSE THAT CAN'T FOLLOW THE OBVIOUS: Responsive Design
postures there being some set of amazing rules able to determine how a
design should correctly respond to being in any variation of screen
size vs pixel density. This simply isn't possible, practical, or even
remotely fun to do. It's faster to custom design layouts and designs
for each screen size and pixel density ratio you want to focus on, and
push that out for each. And you'll get a better looking result.
--- If there's a trick to this, its bandying around the term long enough with your clients that they consider it something expensive to
cover your time. Of course the opposite is true. Designers that
usually throw this around are pretending they can somehow
automagically design once for every device and their secret sauce will
"responsively" adapt their design and layout to all resolutions and
screen size/density variations.
--- Nothing could be further from the truth. It's time to face facts on "Responsive Design"... that it's jargon portending to represent
something impossible or implausibly limiting as both possible and a
good idea, when it's neither.
--- There's just a tonne of work involved in making a site or app work well on all the possible variations, and it involves a lot of
"Standards Compliant" design and development was the last of these fads, before this one.
Think about what it is... like deeply. Responsive Design is an attempt to make any given layout and design idea work on different resolution devices, screens, browser windows and suchlike. It's impossible to make an allowance for all possible variations and all the different fragmentations of device, browsers and screens.
So you have to step back even further, and consider the content, the users and their expectations and the provider of the content and their ambitions.
Now... what devices, screens and browser windows do you want able to see this content, and in what manner?
Grids can be a guide to your initial ideas, but by their rigid nature they're not going to solve the problem completely, just give you a set of "constraints" within which to try out different mutable containers for the content.
The reason very simple layouts and design are popular is because it takes much less work to mutate them into all the various spaces and places they'll be seen... which means it's still a lot of manual work to figure out how you want the content structured, laid out and layered.
So start with all the screen sizes, browser types and their possible logical window sizes, and screen sizes of your app (or whatever it is) and basically build to order. It's that much fun.
btw, the expected margins, gutters, column dividers etc.. they vary from device to device, screen to screen, OS to OS... so it's kind of up to you how you do it. There's not really any rules.
What that video is suggesting is that there's a common (sort of) LPI (lines per inch, you can use any metric you like here) to the experience, so it's just a matter of figuring out a commonality to the screen size. This "SORT OF" works, but you might want to also consider that a phone is held much closer to the eyes, and they have crazy levels of resolution now... etc.
So, no fun at all, really.
Probably the main thing to consider is keeping a visual consistency across all devices, screens and browsers and just thinking of it as a tonne of work.