Are there accessibility guidelines or good-practice standards?

Very novice web-developer here designing a web-site for my new employer (US). I've been asked to ensure the website is accessible to avoid any potential legal issues. I went looking for specs and saw WCAG 2.0 which, to me, was absurd. For videos there is supposed to be captions, a transcript, and a sign language interpretation of the video - which were all rated the highest priority in the specs (AAA)...

From my understanding, the ADA and DOJ never named any specific requirements to make a commercial site accessible. I'm using a CMS (Drupal 8) which claims to be accessible out of the box, but I'm very curious now, what standards do we go by to ensure a good experience for disabled users? Aside from proper HTML for screen-readers and alt-text for media I'm clueless.