HTTPS traffic is encrypted between your browser and the destination server, which raises a reasonable question: what does the proxy in the middle actually see?
The CONNECT method
For HTTPS, most proxies use the HTTP CONNECT method, which essentially opens a tunnel between you and the destination without decrypting anything in between. The proxy relays encrypted bytes back and forth without being able to read them.
What the proxy can and can't see
The proxy typically can see which domain you're connecting to (from the CONNECT request itself) but cannot read the actual content of the encrypted session — page content, form data, or credentials stay hidden from it, assuming the encryption itself hasn't been tampered with.
Why this matters for safety
This is the core reason HTTPS through an untrusted proxy is meaningfully safer than plain HTTP through one: even a malicious proxy operator is limited to seeing which sites you visit, not the actual data exchanged with them.