The first step for my computer to open that link is to do a DNS query, which immediately shares the entire URL (minus the `https://` and the `/`) with my ISP's DNS server (and anyone sniffing traffic along the path, as DNS queries are usually sent in plain text).
https://cf.metaaiusercontent.com/q3-revenue-review-7b1e42f0
No one can guess that URL. The q3-revenue-review-7b1e42f0 part is only ever shared with the Meta's server (protected by TLS).
Now imagine the URL is instead:
https://q3-revenue-review-7b1e42f0.cf.metaaiusercontent.com/
The first step for my computer to open that link is to do a DNS query, which immediately shares the entire URL (minus the `https://` and the `/`) with my ISP's DNS server (and anyone sniffing traffic along the path, as DNS queries are usually sent in plain text).