"De novo" means "starting from scratch", that is: you type the desired sequence into a computer, and the synthesize it.
The "de novo" part is important, because it distinguishes this from the already well solved problem of duplicating DNA from an existing DNA template, which is what all our cells do daily, and which can already be done very efficiently in-vitro with polymerase chain reaction.
Notably, the dream of most of those companies is to have a machine that sits on a lab bench, which synthesises whatever you want.
TODO current de novo synthesis costs.
- 2020-10-05 https://www.nature.com/articles/s41587-020-0695-9 "Enzymatic DNA synthesis enters new phase"