Redirect of subdomain not showing proper page when rule is set to 200

Hello, I’m new to Netlify and I need your help with a redirect issue

My app has a custom domain registered. My site has a page say I wanted to create a new subdomain say which will show the content of without changing the domain in the browser.

But when I set my redirect code to 200 and go to
it shows me the content of, not

But if I use 301! it shows the content of but changes the domain as well to (which is supposed to happen I know)

So, what should I do?

This is my current _redirects*  301!
/*    /index.html   200

We can’t debug or check anything in this placeholder domains. Let us know if you can provide the actual site details to check further.