@media only mediatype and (expressions) {
    CSS-Code;
}

mediatypes:
all    // selects all types 
print  // used for printers  
screen // used for screens
speech // used for screenreaders

Expressions:
(max-width: )            // from width and below
(min-width: )            // from width and above
(orientation: landscape) // landscape mode  
(orientation: portrait)  // portrait mode

Example:
@media only screen and (max-width: 480px) {
    .box {
      flex-direction: column;
    }
}

To apply different styles to different screen sizes.

/* Mobile Portrait */
@media screen and (min-width: 0px) {
           
}
/* Mobile Landscape */
@media screen and (min-width: 568px) {
           
}
/* Tablet */
@media screen and (min-width:768px) {  
        
}
/* Laptop */
@media screen and (min-width: 1024px){  
        
}
/* Large Laptop */
@media screen and (min-width: 1280px){  
        
}
/* Desktop */
@media screen and (min-width: 1440px){
           
}
/* Large Desktop */
@media screen and (min-width: 1920px){  
        
}
/* 4k */
@media screen and (min-width: 2560px){
:root {font-size: 1.4rem;}
}

Simple media query setup targeting all devices. Preferably, use for mobile first approach.

<link rel="stylesheet" media="screen and (min-device-width: 800px)" href="800.css" />

To set different stylesheets at different screen sizes.