An HTTPS proxy is a middleman that passes traffic over an encrypted channel. Sites with certificates work fully through it, and content stays unreadable.
Important: the proxy sees only the destination address, not the data itself. Encryption goes from your browser to the site — the proxy just forwards packets.
For APIs, banks and authorized services this is the best choice: fewer suspicions, stable connection.
When choosing, look at pool freshness and speed. An HTTPS proxy with low ping is indistinguishable from a direct connection.