The "graininess" is a result of having pixels which are too large. You can determine this with a PPI (pixels per inch) calculator.
https://www.sven.de/dpi/
The general rule of thumb (from way back in the print photography days) is that 100 PPI is the minimum acceptable resolution for viewing from a handheld distance. A 27" 1080p monitor is only 85 PPI. A 24" is 92 PPI, so below the standard but close enough that most people don't mind (and we tend to sit a bit further than handheld distance from the monitor). A 27" 2560x1440 is 109 PPI
150 PPI is considered adequate resolution. 300 PPI is considered the limit the human eye can see (at hand-held distances). That's why most printers target 300 PPI as a minimum. (Higher PPI printers exist to make half-toning less noticeable. Toner and ink is either on or off, there is no in-between. So a 600 PPI printer can use half-toning to make 4 greyscales at 300 PPI. A 1200 PPI printer can make 16 greyscales at 300 PPI.)
The names for the different resolutions are pretty stupid, and I've stopped trying to learn them except the most common ones. FHD (full high definition) is the most important one today - 1920x1080. In the old days, VGA (640x480) was the important ones.
https://en.wikipedia.org/wiki/Graphics_display_resolution