So I created a sub domain and added its DNS records to my Cloudflare dashboard. I mainly use Cloudflare for its CDN feature. Is the CDN feature automatically enabled for my sub domain or do I need to do some configuration? If yes, what configuration should I do? If someone could please refer me to an article that explains this, it would be very helpful.
Cloudflare is technically not a CDN, but rather a caching reverse proxy.
As for hostnames - and I presume that is what you referred to - sure, as long as the records in question are proxied () their traffic will go through the proxies and make use of all proxy features as well.