Try it for yourself. Using Chrome 73 or later (you probably already have this), and a mobile browser (either a phone or mobile emulation), try the query [amp dev success stories].
It will only use signed exchanges in Chrome because currently only Chrome supports signed exchanges. The search engine explicitly looks for the browser to state that it supports signed exchanges in an Accept header, like any other new technology.
Yes, any page can use this. So, for example if you went and fetched a signed exchange from https://amppackageexample.com/ (or any other site that supports one, this is just an example), you could then serve that from your own server, more or less just like any other file (the less is that you need to set the right Content-Type header, but it otherwise works just like serving an image or a zip file).
Then, if a user visited the URL on your site https://yoursite.com/cached-copy-of-amppackageexample.com/ then the browser would display https://amppackageexample.com/ in the URL bar, as though that URL had 301 redirected, but without the extra network fetch.
Google search does exactly this, just loading a cached copy of the Signed Exchange, and any other cache (or even any website) can do the same.