Hi, I have started a website with web design and development related but I want to make a sub-domain of entertainment like songs, poems etc so It will be harm for my original domain because that is different theme
No, it is fine, this is what we call as general niche. And nothing wrong if people making website in general niche
Google considers subdomains as different sites. So it is of no harm for your primary site if you are making a subdomain for another site with different category/niche.
Yes they are all different sites, just make sure tu get links from other relevnat saite on your niche
Why you think it is harm? I have use lots of subdomain and it even help some serp and traffic as well.