There are however a bunch of ARIA tags & best practices etc (https://w3c.github.io/aria-practices/) that exist to make popups and dialogs (and other things e.g. tree views or "email-inbox-style" "treegrids" etc) accessible (if implemented correctly).
I am conflicted about these - it is nice that there are ARIA tags for this, but it would also be nice if browsers "understood" aria tags and added some default behaviors (e.g. keyboard navigation). As it is, ARIA tags are essentially "pointless" to anyone who doesn't use an assistive technology, and so non-assitive-technology users nor developers benefit from using ARIA tags so they are often forgotten. If the browsers saw that there was an A11y-tree that matched a treeview or a treegrid, it would be really really nice if they applied some default common keyboard navigation implementation, rather than do nothing and leave it up to the developer to decide what keys do what on each and every site. .... Or on the other hand, is that too prescriptive and should we give developers and UX designers more leeway to design something better, rather than rely on browser-enforced defaults? I guess we are happy with browser defaults for basic inputs, but would we be for a treegrid?