In order to find a particular iframe, you need to find it by its name or src.
Beware, a target iframe must be in the same domain, otherwise its properties and content will be unavailable.
LET page = DOCUMENT("https://www.w3schools.com/html/html_iframe.asp", {
driver: "cdp"
})
LET content = (
FOR f IN page.frames
FILTER f.URL == "https://www.w3schools.com/html/default.asp"
RETURN f.head.innerHTML
)
RETURN FIRST(content)