Today's technique is a great looking floor by Simon NH, that you have to look at twice to realise what piece is being used.
It is in fact a Modified 2x2 Brick with Grooves and Axle Hole, which has been cleverly used upside down, to utilize its unique shape, that isn't shown normally. It can usually be found to represent cargo in some MOCs or even official sets, so this is unique use for an interesting piece.
The bricks can be positioned upside down in a number of ways, however, I think the bricks just sit inside a brick built box, where there are not attached. This allows for a compact design as otherwise the base would be much thicker.
The original technique by its creator Simon NH: https://www.flickr.com/photos/138986803@N03/26180800176/in/dateposted/
The piece on Bricklink: https://www.bricklink.com/v2/catalog/catalogitem.page?P=90258&idColor=85#T=S&C=85&O={"color":85,"iconly":0}