Where do antiviruses get signatures?

Question

Malware

For example, there are free antiviruses without people in the offices who could look for signatures.
Where do they get signatures?

Answer 1

Free - what are they? Which almost everyone has a paid version? Or clamav and the like?
The former certainly have people in their offices, the latter develop like any open-source project, either on the enthusiasm of people (who spend 5/8 in the offices of some large companies), or on sponsors or the sale of a service.