For big units I’ve explained the general approach previously in this thread. Here is a link to the start of the discussion: https://forums.unrealengine.com/unreal-engine/marketplace/31005-advanced-turn-based-tile-toolkit-support?p=1023941#post1023941
As for drag and drop, I cannot give to much input on dragging and dropping in itself, but however you choose to solve it you should look at the Spawn Unit event in BP_GridManager to see how to add new units to the grid. The only important thing is to add them to the appropriate arrays, and if you have any active pathfinding running you need to re-run it for the new units to be picked up.