Which Publii theme are you using?
As an example, referring the demo: https://getpublii.com/themes/demo/artgallery/brown/ , the equivalent amp site is at: https://getpublii.com/themes/demo/artgallery/brown/amp/ – so just add /amp/ to the URL to view the amp site on a PC.
So I guess you could insert a link to the amp version from your main site version (but this shouldn’t be needed: amp-sensitive devices should automatically detect that you have an amp version in place).
For the existing Publii-designed themes at https://marketplace.getpublii.com , I think the key amp coding is already done.
If you’re developing your own theme, check out: https://getpublii.com/dev/ and for amp, particularly https://getpublii.com/dev/amp-global-variable/
Hope that helps.