It can be either mobile first or desktop first (min-width or max-width media queries). However I prefer the desktop first approach because thatās the way Iāve been designing for the last 25 years and the way I am used to. It is also easier for me to have a full screen version and then create a mobile version from that full screen version.
However, thatās my personal preference and designing from a mobile first and upwards approach is good also except that generally (as a freelancer) you are handed the full desktop design first and then have to work out to how to make it work on smaller screens.
Whatever approach you take is fine as long as you are consistent.
I start without any media queries and create the page wrapper and start on the header and navigation The navigation is effectively the most important thing on the page (apart from the actual content of course).
With a horizontal nav you have to ask yourself a number of questions before you start.
How many menu items does the nav have?
Will the items change over time and have more or less entries.
Will the text change on the menu items?
How will the nav cope with a font-size change?
Once you have answers to those problems you can build your nav and then decide at what max-width media query breakpoint you want to create the hamburger menu. Only you and your design will know where this point is and you can do this by opening and closing the browser window until something breaks or looks odd. That breaking point will be where you start forming your max-width media query and take into account any of the questions I listed above.
Once you have done this section and checked all widths and checked in a few different browsers (and validated the html and css) then you can move onto the next section and more or less forget about it.
You then move down to the next element on your page whatever that is and do exactly the same approach.
If you approach each section at a time and test thoroughly you can be sure that everything will work when finished. I tend to keep the media queries just below the section they apply to rather than tacking them all at the end (unless its a simple document) but thatās just a personal preference.
Itās no good designing a whole page and then starting to test and validate at the end as it may be too late to fix by then. Quite often a design will only work if the foundations have been laid properly and fixing something afterwards can be awkward if the structure was not right from the start.
I havenāt ever used the landscape orientation in the last 10 years on the few hundred sites Iāve worked on. Itās really only of use if you are building an app on a specific platform or device.
All landscape heights are different and indeed most users hold their phones in portrait unless they are viewing videos. Even for videos you donāt really need to know about landscape as videos usually have the same aspect ratio and the height will be dependent on the width of the video.
A lot of times users turn their phones to portrait simply because the designer did a bad job of coding the site and they have trouble reading the content in portrait. If designed properly you donāt really need to turn the phone.
If you were thinking you could make a slideshow fit in the height of a landscape phone then that would be quite a task as my phone is only 320px and even if you set the height of your slideshow to match that dimension it means I would have had to scroll my phone to the exact point of that slideshow otherwise top or bottom may be obscured. This is not a viable approach.
Phone users know that they have to scroll so just set a reasonable height where space is limited.
Mostly itās about making best use of the available space.
If at any time you see a horizontal scrollbar on the viewport then that means you have not made the design fluid enough and a device somewhere in the world will die.
A horizontal scrollbar on the viewport is something you need to avoid at all times as far as responsive design is required.
Itās always good to remember also that not everyone has their desktop window maximised. I have a large 27" screen and have 3 browsers and a number of applications open at the moment and all arranged at different size and heights. I still like to be able to read each of those without having to maximise or move the window around. That means that even though my screen resolution is 2500px I am typing this message in a browser window that is toughly 800px x 1000px. In essence you have to cater for an infinite variety of sizes so you canāt do that with an infinite number of media queries.
Create responsive/fluid elements (flexbox and grid are good at this) and then add media queries when the elements are no longer flexible enough to fit.
These articles are old (2012) but explain the concept I just mentioned.
http://tangledindesign.com/deciding-what-responsive-breakpoints-to-use/
https://www.smashingmagazine.com/2012/03/device-agnostic-approach-to-responsive-web-design/
Even in 2012 there were too many devices to account for. There are hundreds more now.