i don’t seem to get the converting of something static (photoshop design) into something responsive
A PSD is a snapshot of a page as one particular size. If the page is never intended to be viewed at anything less that that size then you’re fine and can go ahead and use fixed values.
If it’s supposed to be viewed on lower screensizes (or even higher) then a good designer would/should provide alternative PSD’s for those sizes.
If you don’t have those it’s up to you to decide how want the page to look and react at a different size to your single fixed page design.
How can you possibly make it so that everything is positioned correctly when you have to set things to be responsive…
That’s the fun part. Generally, use % values for widths (even images..which is another discussion) and, generally, everything will scale up and down automatically.
Now, at some point, things will get unreadable so you might have to move elements around so that they can still be read at a reasonable size. There are many techniques that will let you do that.
It’s complex subject and not one that this forum is set up for. That’s for you to discover by doing more reading and research.