I’ve been seeing some videos where people are rolling on whether hits can be allocated to drones or not. I’m very confused, as I thought that it was the defending players choice as to where wounds are designated per unit? What am I missing here?

Nothing, they are wrong.

8th edition codex's Savior Protocol worked that way: on a 2+ the wound could be allocated to the drone and any damage was changed to 1 mortal wound for the drone, shield drone could then take a 5+ FNP.

Edit: corrected the end to read "shield drone could the..."

It depends entirely on if the drone is part of the unit being shot at.

If they are, the defending player can allocate the wounds to models in that unit in any order they like.

If they are not, they can’t chose to put wounds from the attack onto any nearby drones

This is unlike previous editions where drones immediately and automatically became independent units at the start of the game, and could use the saviour protocol rules to intercept attacks on behalf of other units.

You may be watching old videos from the 8e codex