Most of the confusion you are having is between VFR and VMC and between IFR and IMC.
Under Visual Flight Rules, the pilot is responsible for separation from other traffic. This is accomplished by requiring the pilot to be in Visual Meteorological Conditions at all times. This means three miles visibility and specified clearance from clouds. The cloud clearance requirements are to make sure that an IFR aircraft doesn't emerge from a cloud so close that the VFR aircraft can't see and avoid it in time. Under VFR, the pilot need not be in contact with ATC unless nearing or inside control zones such as Class B, C, D airspace. In dark night VFR, the pilot will be using the instruments heavily since there is no visible horizon to refer to. At least in the US, a VFR pilot need not file a flight plan (in most cases) and can use any means of navigation, including reference to landmarks on the ground, VOR/NDB radio navigation aids, or GPS.
Under Instrument Flight Rules, the controller is responsible for separation from other traffic. Because of this responsibility, under IFR, the pilot is always in contact with ATC. This is true whether or not the flight is in VMC or IMC. The controller will use radar or other techniques to keep track of separation and issue clearances to ensure that traffic remains separated and gets to where it is going. When an IFR flight is in VMC, the pilot should be seeing and avoiding also. The pilot will be on instruments in IMC, since there is no other way to keep the aircraft from an upset. They may or may not be on instruments in VMC. The pilot must file an IFR flight plan and be cleared for a route by ATC and may be re-cleared as conditions develop over the course of the flight.
A visual approach is not an Instrument Approach Procedure. It can be given when the pilot is in VMC and reports the airport and the traffic they are following in sight. The pilot will fly using visual cues from being cleared for the approach all the way to touchdown, but they are still under IFR.
On an instrument approach, the procedure itself is responsible for terrain avoidance above Minimum Descent Altitude/Decision Height. If flown according to the charted approach with the appropriate altimeter setting, the aircraft is guaranteed obstacle clearance down to MDA/DH. If flown with controller issued vectors, the controller remains responsible for terrain avoidance and will do this by using Minimum Vectoring Altitudes. When you get to MDA/DH, you must have the "runway environment in sight" (one of a dozen or so things are acceptable) and from there you would complete the landing using visual cues, and once again you are still under IFR. If you don't have the runway, or lose it before touchdown, you must initiate a missed approach and fly the published missed procedure until ATC issues a different clearance.
There are many little exception cases in what I've outlined.
I hope my English is understandable and please ask further questions if you want.
If possible, I encourage any ATC student to take a flight and experience these things from the pilot perspective.