My reply from the other forum:
Those two latches on either side of the trunk...release the upper rear seats so that they fold down. I'm surprised they haven't fallen down if you released them.
If you don't have a physical (paperback) owners manual...you can simply type
"BMW 2018 6 series owners manual PDF" into a google/internet search box and find an owners manual you can download to any device of your desire. If you can't find a "2018" manual...if you can find a 2016-2019 version...it will be exactly the same.
And if you have none of the manuals that came with the car...you can do the same for the
2018 BMW Navigation Entertainment Manual...it will go into details about iDrive etc.