When you know code good enough and you can write code and visualize in your mind what that screen looks like with the written code that is always the best designer, therefore the keyboard.
desktop , tablet and mobile design
I am guessing you are talking about mobile-friendly vs a mobile app.
If that is the case then proper media queries.
Even if you use bootstrap I'd still learn this stuff manually prior to using any Library just so you have the skill.
I feel that if someone is going to be a developer they should know the underlying skills without a library first.
Maybe an oldskool answer to help you on Your jouney.
Please use the slice tool in PS. If you can slice it in, Let say square parts you should able to code it pretty easily. My advice is not to use the actual slices, but you will notice the importance of a (web) design. Of course there some nifty css options, to make it more fancy and create rounds and radius. But that’s advanced. If you cannot slide it, then you need to use the image as a background and make the image responsive.
FYI, responsive is all about grids and the layout. I think you designs are not easy to put in a grid. That’s way I made my slice remark, and start with a mobile first design first
Also try design you image in a mobile template in PS.. And solve some issues over there.
If you write your own CSS, you really need to look up media queries, that's how to also make a site mobile friendly. Media queries are fairly easy to deal with.