Yes. ILS simplified is kind of an easy concept which is often made overly complicated. It is a system that has antennas at the end of the runway which transmit a signal that is – for practical purposes – a very narrow signal which the aircraft can use to determine if the AC is on course inline with the runway. That is the LOCalizer aspect.
Glideslope (GS) is similar, but beans a vertical signal which the aircraft can use to determine if it is descending on the glideslope.
ILS typically implies LOC and GS. If GS is not provided, it is usually noted as LOC approach.
To use ILS you need and aicraft equipped with the receiver and the indicator called a CDI (course deviation indicator)
The CDI will have lateral (side to side) needle that shows if you are on centerline, and if not which direction and how far you are off. It has a similar needle for GS – it indicates if you are on the GS, and if not if you are above or below and by how much.
You don’t “need” a flightplan or to use autopilot t fly an ILS approach. But you need a way to get to the right place to start. There is a navigational waypoint called an IAF (initial approach fix) which you go to that point and turn onto the ILS heading and then use ILS to follow centerilne and GS. These days, aircraft use GPS to get to that IAF. Otherwise, you can ask to be vectored to it.
Where the flight plan comes into play is that it involves getting you from your destination to the destination IAF in a panned, proper manner. In IFR flight, you plan and file that route, and technically if you and the aircraft are certified for it, you fly the whole thing with no visibility all the way down to a few hundred feet over the destination runway.
But the conditions don’t have t be IFR to fly IFR, and you don’t have t be flying IFR to use ILS. Although, if conditions actually require IFR you would been to be certified ti fly in those conditions.
The autopilot has an APProach mode and can follow ILS. For that t work, you need your CDI set to LOC1 or LOC2, the ILS frequency in the appropriate nav radio(s), and course (CRS) set to the ILS runway course.
You are supposed to intersect GS from below for best results. That ius usually the reason AP will fail to follow it.
In terms of lateral intersection, usually you intersect with about a 20 degree intersect path. Of course, the narrowness doesn’t matter too much, but a lot of autopilot systems won’t engage APP mode if you are not on an intersect course when you press that button.