@wheattruth,When you are trying to work with SPAs in AEM, Angular9 is
being utilised with projects and examples online, you can find some
example code here:
https://experienceleague.adobe.com/docs/experience-manager-65/developing/headless/spas/spa-getting-started-angular.html?lang=en
However, when you are creating a new client library as your front-end
application, you can choose any Javascript library or framework of your
choice.