Do iframes reliably protect their content from parent access? (inverse frame busting)

Question

since search engines or at least my search bubble is polluting the keywords "iframe", "security", "access" and such with answers regarding escaping from iframes as in "protect yourself against iframes", i would like to ask here:

• Do iframes securely isolate the iframe's internals against access by the parent?

• If a secret is used in an iframe, can it be exfiltrated?

Thank you for your answers.