These use X and Y, I can imagine on a controller this inventory system working very well, but on keyboard it's a bit of a pain to start with. Holding down the key for a gun, which would be 1 or 2, allows you to do two basic modifications - remove/add light or remove/add suppressor.
You can do several things with the inventory, but every PC gamer knows how to switch weapons, so I am going to talk about modification of those weapons. Kill - Simply press E to kill and done.Knockout - Simply tap the LMB, it's slightly counter intuitive to let of the button, but not difficult to pull of.When you have finished you questioning you let go of Q and quickly hold LMB again to get back to the initial options. But be sure not to let go of Q otherwise you will let go of the prisoner. Interrogate - You press Q to initiate the interrogation, now you have to use the arrow keys to select the question to ask.You now have a choice of what to do, interrogate, knockout or kill.
To initiate a grapple you hold LMB, when near an enemy, with no weapon drawn. The controls become fiddly when dealing with grappling or using the games inventory system. One thing which is worth noting is that the mouse does not work for the menus, not an issue as arrow keys or WASD keys work just fine.