If the player does NOT assign a settler to a task, what does he do? Stand around all day and night? Or self-appoint himself to some useful task like farming or guard duty? Do any unassigned settlers take it upon themselves to plant new crops? (A serious consideration if the settlement grows to the point where there is less food than there are settlers.)

they scrounge for random junk. like the table but supposedly the table just makes them better at it.

The only ones i assign are the guards and store clerks, everyone else finds work on their own. Same with the beds, they pick it on their own.

I've been wondering about that because I was told I had a bed shortage despite having more beds than the supposed max amount of settlers allowed in the settlement. I looked and had several unused beds that I wound up having to assign people to manually.

One thing this game could use is a better way to manage settler jobs.

They will be assigned to food production first, then they will do other duties including defend the settlement (but this appears to be a rare thing) and scrounging but will only use the table if told.

If you add more food after they arrive they wont reassign to food production but a new settler will start working on food if there are food plants with no one assigned to them.