If I worked in the United States all of last year but earned a small amount of dividends from Canadian investments, what exactly do I need to file?

You should contact the Canada Revenue Agency (CRA) to double check your residency status, and whether you need to file a return or not. If you are in Canada or the United States, you can call the CRA at 1-800-959-8281. If you are elsewhere you can call 613-940-8495. This page has more ways to contact the CRA: https://www.canada.ca/en/revenue-agency/corporate/contact-information/telephone-numbers.html

If you have TurboTax Live Assist & Review, here is how to talk to a Tax Expert: https://turbotax.community.intuit.ca/community/navigating-turbotax/help/i-m-using-turbotax-live-assist-review-how-do-i-talk-to-a-tax-expert/00/873764