Can I link a windows laptop to iMac.

No, iMacs are computers they are not external displays for other devices. Target Display Mode (TDM) was discontinued in 2015, 6 years ago now and only worked on a small subset of Mac to Mac. Late 27" 2009 and 2010 iMacs would work with PCs.

You are best to simply buy an external display for your PC. This the most effective and most reliable solution. You can find external displays beginning at $100.

No. Target display mode only works between Macs.

Also a 2020 iMac running Catalina or Later does not support Target display anyway.

To share files, sure! What do you want do when you say "Can I linK A WINDOWS laptop to iMac?"