normally when you buy a domain in other place
Chances are, that other provider you used is both, a domain registrar and a hosting provider (GoDaddy, HostGator, ..etc as an example).
#I bought a domain in cloudflare first time but it is now saying : "Pending Nameserver Update"
prime nebula
Cloudflare will not host your site (unless it's a static site, which you can use Cloudflare Pages for). But if you're running servers or anything which is not static, you'll need to host it somewhere, then you may use Cloudflare to serve end-users without exposing your origin server.