How to get free domain like .com.nu like that ?

Com.nu website registration is not working

You can use Freenom - they don't have .com for free, but they do have .ml, .cf, .ga, and .tk for free. Literally takes 5 minutes to register a domain there. If you can spare $1, there are many $1 domains - the one I have in my bio, I bought for $1/year - IONOS and NameCheap are some of the many that have $1 domains. Typically you can get some .net, .co, .xyz, and if lucky .com as well for just $1 or less.

.com.nu is a swedish domain. Think they're available on Namecheap.