This is the relevant law: https://www.buzer.de/8_TDDDG.htm (It used to be §90 TKG.)

It's specifically about bug-like devices which transmit the recording. A typical consumer camera does not provide simultaneous uploading, I think. A device that is just recording for storage is not regulated this way.

The law has been applied to baby monitors disguised as toys, which makes some sense to me. Smart watches and phones where the microphone can be controlled remotely, with that capability being marketed specifically for monitoring purposes, have also been targeted.