A:Answer It won't. The 4G LTE on the Pixel 3a goes up to UE CAT (user equipment category) 11 on download (600 megabit download) and CAT 5 on uplink (75 megabit). According to AT&T's Internet of Things website, the highest it's rated for on downloads for AT&T currently is CAT 9 (450 megabits).
There are some LTE categories that are much faster but I don't think the Pixel is upgradable to those. It certainly won't be upgradable to 5G which operates on much higher frequencies than anything supported by the Pixel's radio.
A number of the LTE categories (the very fast but non-smartphone categories 8 and 14, and categories 16 through 20) are gigabit or higher speeds though, and there are already 4G phones that support those (e.g. the Samsung S10 models on the two gigabit CAT 20, the fastest that I know of regularly used on a smartphone, and the iPhone XS and Google Pixel 3 on the one gigabit CAT 16).
