Correct, caching is automatic. It takes a visit for that person’s local datacenter to cache resources. The next local visit will get cached resources until the cache expires or is purged/evicted. Each datacenter is an independent cache.
How often is the cache of my site updated or is there a specific trigger to it? Like for example content sites, their content is changing constantly, so how often is their cache updated compared to my low traffic website?
Cloudflare doesn’t cache HTML by default, so any content changes show immediately. But if you modify CSS or JS, I suggest you use the Cache section of the Cloudflare Dashboard to Purge Everything.
DNS doesn’t change, so there’s no need to purge anything at cloudflare-dns.