Most accessibility shops and devs tend to use the free NVDA screenreader[0] for accessibility testing because of the costs associated with JAWS. There are differences between the two (and the other well used screenreader VoiceOver) but NVDA is a damn good product with a decent user base that certainly helps in finding potential hiccups in complex frontend applications. Usually this happens when one of the screenreaders hits something on a webpage that triggers 'interaction mode'[1]

Edit: Interestingly the most recent data I found[2] seems to indicate that NVDA has passed JAWS as the most used screenreader, so might be worth using as the default for testing

[0] https://www.nvaccess.org/about-nvda/ [1] https://tink.uk/understanding-screen-reader-interaction-mode... [2] https://webaim.org/projects/screenreadersurvey8/