Aim with the mouse (or gyro-as-mouse), fire with your gamepad.
Hold your phone sideways like a gamepad. Aim by moving it, tap the right side to fire.
Gamepad fire clicks whatever is under the crosshair, so the whole menu works from the controller. “Right stick” moves the crosshair at a speed set by how far the stick is pushed; the mouse still works alongside it. Start = start / pause, X = reload, Back = menu, Esc = pause. “System” uses the real OS cursor (lowest latency, but stops at the screen edge). “Pointer lock” draws its own crosshair from raw relative motion so it never hits the edge, at the cost of a little display latency. If Steam Input maps your trigger to a mouse click, keep “Mouse click fires” on.
Tap the right half of the screen to fire; hold your finger down in tracking and weapon modes. Hold the left half to switch the gyro off, so you can recenter the phone (ratchet) without moving the crosshair. Tap the top bar to pause. Reloads are automatic. “Pointer” aims where the phone points: turn left/right, tilt up/down, at any holding angle. “Steering wheel” rolls it for left/right instead. If the crosshair moves the wrong way, use Invert.