I have done my redirection from .htaccess all the things also are working correctly but when I am checking elsewhere like seo checker or Google in-sight there is a warning written WWW and non-WWW work as separate websites this everywhere how can I fix it. If possible I want to make the (www and non-www) or (http and non https) site as one single website
Yes, of course they are. The domain arvat.rf.gd is hosted with us and www.arvat.rf.gd is hosted with Google.
If this is not what you want, then please:
Migrate your website to our hosting.
Remove the CNAME record for the www subdomain.
Remove the domain from your account and add it again to fix the www DNS record in our namesevers.
Hosting our subdomains with other hosting providers is not supported. If you choose to hack around this, then know that you do this at your own risk. We make no guarantees that it works well or at all. And even if it works now, relying on bugs to build your website means it may stop working at any time.
For example, you could try to not hack around the limitations of our subdomains, and either use subdomain provided by Google, get your own domain name, or find a subdomain from a provider that actually supports hosting a website elsewhere.
I think I know why our subdomain without www would be detected as another website. And no, you cannot change that. And you also cannot host the base subdomain elsewhere.
I honestly don’t think there is a way to fix this issue while still having this exact subdomain hosted with Google. But if there was, I still wouldn’t tell you, because you’re not supposed to do this with subdomains.