Both Dynamic Search Ads (DSAs) and Responsive Search Ads (RSAs), are automation tools for search ad creation.
The key difference is that RSAs are a partial automation whilst DSA are full automation.
RSAs allow marketers to create multiple headlines and descriptions for their ads. Google then combines this ad copy and automatically tests for the best results. Google takes into account search queries, device use, and other demographic information to determine which combination of copy will drive the greatest clickthrough for a given search. The longer marketers run these ads, the more data Google accrues to further optimize RSAs.
DSAs differ from RSAs in that Google completely automates the creation and distribution of the paid search ad. With RSAs, marketers create the ad copy and the keyword list. That is not the case for DSAs. Google crafts the ad copy by scraping content from the client’s website, either from a specific landing page, category page, or the entire site. They also use this information to build out the keyword list.