If you are receiving a classified briefing you cannot be on a machine that has internet access - the briefing would be in a secured area with no personal devices and the only machines in that area are airgapped (and they are airgapped forever, no switching back and forth).

You could of course write stuff down afterwards in an unsecure place but that is day 1 essential huge fucking deal no no. You don’t even discuss classified info outside a secure area, not in your public office not in the outback not ever. That doesn’t mean people don’t do it but when they do and it is found out it is a really big deal. Accidents do happen and there are protocols in place to deal with them when they occur. 99.9% of these leaks are extremely mundane low tier classification and are due to document misclassification etc. Sometimes the name of a project is classified and is leaked by reference etc but when it comes to actual important stuff people are quite competent at keeping that in secure areas.