When assessing a new ranking signal Page Experience, the launch of which is expected in 2021, Google will look at the AMP-version of the page. This was said the search employee John Mueller during the last hangout for webmasters.
Therefore, if the site introduced AMP support and he was transferred to the mobile-first indexing (which will be included for all sites by the end of 2020), then Google will analyze the AMP-version rather than the desktop or the mobile page.
As explained Mueller, usability evaluation will be based on the page that the user sees after the transition from the search results. If your site has implemented AMP technology, users of mobile devices will see an accelerated version of the page. Accordingly, it will be Google and evaluate it.
Recall that the search engine has announced a new ranking signal Page Experience at the end of last month. On the Google Developers site is already available guidance on it, including in Russian.
In the Russian version of the document, this signal is called "convenience of the page."