e.g. I have that Facebook Container, and I also have a Slack container I just put together in the usual way by opening my Slack session (it's for the main social group I hang out with, during the pandemic) inside a Container with a pink theme and icon.
Suppose three friends send me a funny Youtube video of kittens, one sends it on Slack, one on Facebook, one literally sends me a postcard with the URL on it.
In Facebook, it's inside the Facebook container. Since the Facebook Container has no idea who I am, Youtube presents adverts and of course there's no way to add the video to my "Fun kitten videos" list. But if I tell Youtube to open this now the tab is not Facebook, a no-referrer link opens with the URL and now in my default context which has Youtube Premium, so there are no adverts and I can add this to my lists. As far as Facebook is concerned I apparently just left. Unless Google tells them I watched that video they are none the wiser.
Slack is inside the Slack container. So again, no Youtube account, adverts. But if I open the Youtube page that's still inside the Slack container, so still no Youtube account. I need to explicitly get the URL and paste it into a not Slack tab to get my default context.
The link from the postcard obviously I get to choose which context to type it into the URL bar, although maybe the UX of typing random Youtube URLs in isn't great.