They do what is called "genotyping"[1] which is much cheaper than full "sequencing" [2], but which only probes for a limited set of known variants. So it's only partial information.
Since then 23andMe has launched a more expensive 23andMe+ Total Health offering, which does full sequencing, but like most people, I never subscribed to that package. In fact I had no occasion to interact with the company after the first result, which may be why they are struggling financially.
DNA testing is something most people might do at most once in their lives -- then they lose interest.
[1] https://customercare.23andme.com/hc/en-us/articles/202904610... [2] https://customercare.23andme.com/hc/en-us/articles/202904600...