First, i flashed e/OS to test it. After that i noticed, i was able to ignore “Signature verification”.
So i flashed the image using this command while running sideload in recovery mode:
adb sideload /Users/jochen/Desktop/LineageOS/12-12/lineage-20.0-20231212-UNOFFICIAL-FP5.zip
Regarding the partitions, that confused me as well. In LineageOS i used to flash a single zip file on my old OnePlus One. Maybe that depends on the device. Would be great to have a single zip file to flash the OS. But dunno if it is possible at all.