devices - iCE40 FPGA devices. The devices architecture descriptions have approximations of the real fabric, to get real fabric you have to override the rr_graph.xml file.
devices/layouts - Tile layout descriptions for devices.
devices/tile-routing-virt - Version of architecture which uses;
devices/top-routing-virt - Version of architecture which uses;
primitives/ - The primitives that make up the iCE40. These are generally used inside the tiles.
tiles/ - The tiles found in the iCE40 architecture. The iCE40 only really have 3 tile types,
tiles/plb - Logic tiles, called PLBstiles/pio - IO tiles, called PIOstiles/block_ram - Block Ram tiles, which don't really have a name.