It’s actually not a complicated issue. Some hosts like FlyWheel (https://getflywheel.com) will let you launch a new instance of WordPress using a temporary domain, like https://chosungmin.flywheelsites.com and you can then build your new WordPress website there and when you are finished you just switch over your DNS settings to point to their new IP.
This is the easiest way to do it – just build somewhere else with a different, temporary domain and then when you’re ready, switch it all over.
You could also just add a sub-directory to your current website… …so if your site is https://chosungmin.com, you would build your new site in https://chosungmin.com/new-site/
And then when you’re done, you can just make a redirect or you can move the whole thing to the root directory.
There are lots of ways to do this.