23andMe and similar companies don't transcribe your entire genome because that would cost way more than they charge you. They just sample a few tiny sections of it.

Probably not any 23andMe haven't already told you about. They test a limited set of SNPs, balancing between ones thought useful for genealogy, ones useful for ethnicity estimates and ones thought useful for health-related things (the latter they would like to make their main selling point, the two former are really all commercial DNA services' bread and butter).

It's unlikely that they would luck into testing some unknown SNP which turned out to be relevant for disease.

23andMe tests SNP's (single nucleotides) that are inferred to be significant in protein function/epigenitics.

Those SNP's i believe are testd from primers

so what 23andMe does is specifically on the back of previous research and afaik their data isnt technically clinically significant as most findings need confirmation or more tests.

Not really, no.