Please find this might help you
First you need to enable it
command: php bin/magento config:set dev/css/use_css_critical_path 1
or by admin panel: Store > Setting > Configration > Advance > Developer > Css setting > Use CSS critical path change it to enable
Then you need to add critical.css file ( Might get error ) app/design/frontend/<your_vendor_name>/<your_theme_name>/web/css/critical.css
Now what is the use of it - easy way whatever we see on site on without scroll that CSS should be added over here
Why it is useful? - When we use CSS merge that file become heavy to get download and make TTFB take more time to load because of critical CSS that CSS file loses its a priority to get a load with HTML and push it down to get load after HTML make user interactive fast.