Every md member carries a superblock recording the array's UUID, level, chunk size and layout, the member's role, the data offset, and an event counter incremented on every write to the array. At assembly, mdadm groups members by UUID and starts the array from the members whose counts agree. A member whose count is lower missed writes, because it dropped out during a power cut or a cable fault and came back later, and its copy of the data is older than the others'. The kernel says so, kicking non-fresh, and assembles without it. If that leaves fewer members than the level needs, it says Not enough devices to start the array and leaves the array inactive. Nothing has been written; the state on the members is exactly what it was.
--examine reads a member's superblock and prints all of it: Version, UUID, Raid Level, Raid Devices, Data Offset, Super Offset, Layout, Chunk Size, Device Role, Events. Run on every member, it is the whole case, and it writes nothing. --assemble --readonly starts the array without writing to it. dmesg and /proc/mdstat report what was tried. Those are the diagnostics.
--assemble --force tells mdadm to include a stale member as if it were current, and the array then runs with weeks-old blocks in one member's stripes and writes on top of them. --create writes brand-new superblocks on every member with a new UUID, the level, chunk and layout typed on the command line, and the data offset the current mdadm version uses; with --assume-clean it skips the resync, and without it the resync rewrites parity from the data as the new geometry sees it. --create does not write to the data area, so the data survives it; what is lost is the description, and on an array created with an older mdadm the data offset, so that the superblock says one thing and the partition table sits somewhere else. The bench recovers the geometry from the parity and the true offset from where the file system actually starts, and the old superblocks are sometimes still readable at a different location from the array's earlier life.