Regular pages and AMP pages are linked through a snippet that is placed on the regular page. You can find this on the overview screen of the AMP page.
An important thing to note regarding linking is that if you navigate directly to the AMP or regular page URL, you will be served the respective content regardless of whether you are on a desktop or mobile device, i.e. if you navigate directly to a regular page link on a mobile device you will be served the regular page and not be redirected to the AMP version.
So how does AMP linking work?
AMP has a list of supported platforms that can be found here. When a page is linked from one of these platforms the platform makes a decision on whether to serve the regular page or the AMP version.
Let’s look at an example. Below are links for a Guardian article’s regular page and respective AMP page.
If you navigate directly to either of them you will be served that content regardless of the device you use.
However, if you click on the same article linked from Twitter (a supported platform) you will be directed to either the regular page or the AMP page depending on your device. You can check it out here. Click on the link from the Tweet and check the url of the page that opens. It should be the regular page on desktop and the AMP page on a mobile device.