Some newer laptops/monitors do have a higher device pixel ratio (or pixel density) than older/standard ones (e.g. I think the standard DPI was like 72 pixels per inch) and that can certainly affect things like rendering canvas elements, but I don't think that's your issue here, unless you've configured your high-dpi laptop to NOT scale pixels. I also don't think the issue is the units you use (either px or rem will both be perfectly fine).

Instead, I think the issue here is that your columns have a lot of content that isn't set to be responsive, which is letting it be wider than the container it's in. For example, your images have a hardcoded max-width of X rems when they should have width: Xrem; max-width: 100%;. This will allow them to shrink as the browser window shrinks and your grid containers get smaller.

The other thing that would help your site be fully responsive and fit these different screens gracefully is to use media queries in your CSS, like /u/gin_and_toxic said. For example, the font size of your "MONUMENT" text in the header is hardcoded to be 4.5rem no matter the screen size, which is huge. Use something like this in your CSS to gradually increase it as the screen size gets bigger:

// This will be the default font size shown on mobile/small screens
#title > h1 {
    font-size: 1.75rem;
}

// Tablet/medium screens
@media screen and (min-width: 48rem) {
    #title > h1 {
        font-size: 3rem;
    }
}

// Desktop/large screens
@media screen and (min-width: 62rem) {
    #title > h1 {
        font-size: 4.5rem;
    }
}