Use a spudger to loosen the right speaker cable's connector from its socket on the logic board.
It is useful to push downward on both short sides of the connector to "walk" it out of its socket. Be careful with the corners of the connectors, they can be easily broken off.
Pull the connector downwards to remove it from its socket.
Pull the right speaker straight up about an inch, toward the top of the iMac.
Lift the right speaker straight up and remove it from the iMac. This may take some force, both hands and rocking the speaker right and left to get it out.
Tilt the top of the logic board away from the rear enclosure.
Lift the logic board straight up and out of the iMac.
Be careful not to snag the board on any of the rear case's screw posts.
The I/O boards at the bottom of the logic board will be the greatest challenge. It is recommended to pull gently to avoid any damage.
During reassembly of the logic board, pay attention to the position of the I/O connectors. When the board is back in the case, insert a USB or Thunderbolt cable into one of the connectors to align it perfectly.
It is a special blade SATA drive, essentially just a SATA SSD with special connector. Same form factor as in 2012-2013 MacBook Pro Retinas. You can install a normal M2 SATA (not NVMe!), but with an adapter
Remember guys: the base model that did not ship with SSD is completely missing the M.2 connection soldered on the board from the factory, and will not be able to mount one. The main HDD can still be swapped for one though.