SEM search results have ad extensions. SEO search results have featured snippets. When comparing SEM vs. SEO, you’ll also find differences in the appearance of the search results. SEM search results may include ad extensions, which can add on additional links, phone numbers, and callouts. On the other hand, SEO results may appear with featured snippets in search.
The caveat in all of this is that, in one way or another, most of the data and the rules governing what ranks and what doesn't (often on a week-to-week basis) comes from Google. If you know where to find and how to use the free and freemium tools Google provides under the surface—AdWords, Google Analytics, and Google Search Console being the big three—you can do all of this manually. Much of the data that the ongoing position monitoring, keyword research, and crawler tools provide is extracted in one form or another from Google itself. Doing it yourself is a disjointed, meticulous process, but you can piece together all the SEO data you need to come up with an optimization strategy should you be so inclined.
An SEO audit should not be rushed. It simply takes time to uncover root causes of the issues affecting your online health. Depending on the size of your site, a proper audit can take anywhere from 2-6 weeks to complete. Due diligence is required when making major changes to any website, and an SEO specialist must conduct a thorough investigation to make accurate, impactful recommendations.