ADS-B OUT on 1090ES by aircraft are typically sent by a transponder in the DF17 message.
There is a separate by quite similar DF18 message used:
- by airborne non-transponder ADS-B OUT devices (more and more of those coming into the market; don't yet know if they fulfil the US mandate)
- by ground, to uplink SSR-traffic, as TIS-B
- by ground, to uplink ADS-B traffic from other data links (mainly UAT), as ADS-R
Does OpenSky process also DF18?
How do I know if DF17 or 18?