When buying an item, how can I assign it to another squad member instead of Shepard? How do I swap inventory from Shepard or another squad member to a different character?
You can equip teammates when they are members in your party during missions, or in the bay where the Mako is stored (lowest floor) on the Normandy by using the lockers.
Each member in your crew has a locker in that bay. The squad lockers are on the wall on the left side by Ashley Williams. Find the locker corresponding to the character you want to equip and change their equipment.