The details shared here apply to the most recent version of the Artisan theme. To keep your theme up to date, check out this helpful document.
The Artisan theme's product page features a "sticky" add to cart button when viewed on mobile. This sticky add to cart button will only appear when the "dynamic checkout button" (buy it now button) is disabled.
If you would like to enable this feature, you can disable the dynamic checkout button by heading into Online store > Themes > Customize > select "Product Pages" in the drop-down page selector. Under Sections > Product > Uncheck "Show dynamic checkout button" and Save.
Once you've completed this, if you view your product page on mobile, the add to cart button will become fixed to the bottom of the page on scroll.
Alternatively, if you're looking to disable the sticky add to cart button on mobile, you can enable the dynamic checkout button instead.