(To anyone more familiar with these concepts reading this: yes, I am aware that this is a simplification. That's the point
X.509 is a format or "standard", describing what a digital certificate looks like and how one may create one, so that applications, operating systems, etc. may implement them in the same way. You can think of it a bit like how .jpeg or .pdf are file formats, which describe exactly how the data in them is stored, so that any application implementing this format can display the pictures or files the same way!
X.509 is very very old (published in 1988), much older than Cloudflare or LetsEncrypt. Cloudflare and LetsEncrypt do offer SSL/TLS certificates, but they are by far not the only two vendors doing that—comparable to how there's also more choices than BMW and Mercedes if you were to buy a car
