Changing a switched outlet to half-hot and found pigtails
Easy one. Here are the secrets:

The circuit involves a switch loop which uses /2 cable (black-white). Switch loops don't have neutrals.
If white is not used as a neutral, it must be used for the always-hot, not the switched-hot (so it reliably reads hot when touched with a voltage tester).

So... 1 white among blacks tells us a switch loop is involved, and the bundle is certainly the &quot;Always-Hot bundle&quot;.
That being the one you need to add your always-hot pigtail wire to.
All these wires have other jobs to do, and so they must remain together.  They are not &quot;spares&quot; for you to pluck one out of. Any end scenario must end with them all still together (plus whatever wire you may want to add).
You'll need to obtain a pigtail wire from any shop that sells wire by-the-foot.  Solid white 12 AWG THWN wire (&lt;25 cents a foot) is the universal donor, since it works in 15A and 20A circuits, and can be taped to make it a hot.
DO NOT obtain one by cutting short any of the wires in the box.  They don't have length to spare, they have statutory minimum lengths they must have, and you don't have enough excess length to make a pigtail.

Code now requires that a white wire used as a hot must be taped with black/color electrical tape to indicate its use as a hot, so now is a good time to correct that.