Google Forms
Online surveys are online forms and yes they need to be ADA compliant whether embedded in the site or linked to from the site. Typically if you are using Google Forms or Survey Monkey it is my understanding they have the fields in their forms so you can make them ADA compliant. Though, you might find this Goggle Support link useful https://support.google.com/accessibility/answer/1631886?hl=en and scroll down to the Google Forms section. We do not support 3rd party widgets.
However, do be aware Siteimprove does not crawl 3rd party software so will not flag these forms. You must ensure they are ADA compliant manually outside the tool.