Cloudflare knows almost everything done from your IP address because they're used by the majority of websites. And some websites are using a cloudflare signed TLS certificate so if cloudflare wants, can see the content of the communication instead of an encrypted package
So they know if you have a human behavior (visiting many different websites at human speed and having rests during sleeping time) or if you have a bot behavior (sending millions of requests to the same endpoint at superhuman speeds)
There's a default setting that allows unencrypted communication between the server and cloudflare. So they receive unencrypted data, sign with their certificate. Or send with self signed certificate, they decrypt and reencrypt. Or for some reason can download and import on the server their own internal use certificate.