2001 Nature article speaks to how the arms are brains+actuators:
By stimulating nerves in amputated octopus arms, or just by tickling the skin near them, Binyamin Hochner, of the Hebrew University of Jerusalem, and colleagues made the limbs reach outwards just as an intact octopus reaches for a piece of food.
“We saw autonomous control of the entire arm,” says Hochner. This and other simple experiments demonstrate that the nervous control program for reaching is written into the arm’s nerves, the team believe.
2008 Telegraph article highlights a theory that there are 6 “arms” and 2 “legs”:
A study by scientists at Sea Life centres across Europe found that the invertebrates move across the sea bed using their two rearmost limbs, leaving the other six free for the important business of feeding.
Researchers who observed the creatures in action found they push off with the “legs” and then employ the other tentacles to pump themselves along.
This revealed the surprising simplicity of their motion; they just choose which arm to use to push themselves along.
Apparently all arms exert the same amount of force and they move completely arhythmically.
2019 Science Daily on how it is proven that the brain isn’t needed for the arms to communicate with each other:
“The octopus’ arms have a neural ring that bypasses the brain, and so the arms can send information to each other without the brain being aware of it,” Sivitilli said. “So while the brain isn’t quite sure where the arms are in space, the arms know where each other are and this allows the arms to coordinate during actions like crawling locomotion.”
Of the octopus’ 500 million neurons, more than 350 million are in its eight arms. The arms need all that processing power to manage incoming sensory information, to move and to keep track of their position in space. Processing information in the arms allows the octopus to think and react faster, like parallel processors in computers.