The vast majority of aircraft (including the GA aircraft you named) have a center of gravity (CG), which acts downward toward the earth, and is in front of the center of lift (CL), which mostly acts upward. This alone means the airplane is producing torque that constantly wants to tumble nose down (tail over nose). The horizontal stabilizer/elevator combination (we’ll call it “the tail,” even though we’re talking about the horizontal portion only for these purposes) counteracts that nose down torque by normally providing lift downward, forcing the tail down and the nose up.
When in steady, stabilized flight, the amount of downward lift the tail is providing is balanced so the nose doesn’t move either up or down at that moment. However, as lift is basically a component of angle of attack and velocity, if something changes, like speed or power setting, then it will no longer be in trim.
Specifically, if you’re trimmed for level flight and pull the power, the airplane begins to slow, the downward lift on the tail decreases, the tail rises, and the nose falls. As you start to descend, the combination of aerodynamics and gravity begin to increase the speed, and as it approaches the original speed at which the tail was trimmed, the tail down force increases and the nose starts to come back up. If you have the altitude, time, and eliminate other factors like bank, eventually the airplane will settle at or near the original speed at which it was trimmed. But now, without that extra power in, you’ll be in a steady descent.
The opposite would happen if you were trimmed level and added power.
Now, say that you want to reduce power and descend, but you don’t want the nose to drop, instead you want to keep the same pitch attitude. You’d have to hold the yoke back as the aircraft slows and elevator loses that effectiveness. The airplane is going to eventually begin to descend (otherwise it will stall). At this point, once it’s in a descent at the desired slower airspeed, if you were to let go of the yoke, the nose will drop and the airplane will want to fly at the original airspeed at which it was trimmed. But without the extra power in, you’ll be in a descent.
However, instead of continuing to hold the yoke back, you can use trim until the yoke is hand-off neutral and the descent is stable. The airspeed should be stabilized and this is the new airspeed at which the tail is balanced. At this point, if you were to add power back in the nose is going to pop up and the airplane is going to go through oscillations until it settles at or around that newly trimmed airspeed, which with all that power back in means you’re climbing with the nose high.
If you want to restore level flight, you can push the yoke forward, now the speed is increasing and you have to increasingly push forward. Once things start to settle at the desired attitude and speed, you trim again so you don’t have to hold the yoke forward.
All of this is part of an aerodynamics demonstration that you might do in one of your first several lessons in an aircraft. The nice part of the sim is you can do this all on your own for less than the cost of a single flight lesson, and no worries about crashing.
TL;DR, the trim relieves the pressure necessary on the yoke by balancing the tail up/down motion at a certain pitch and speed (and power) combination. Change any of those elements, and you’ll have to re-trim.
As for the question about descending, there is an axiom that pertains to many aircraft: “you can go down or you can slow down, pick one at a time.” The best thing to do is plan ahead for your descent by understanding the distance you’ll need to descend. Knowing the normal rate of descent and normal descent speed can help with this (you can just do math), but generally you can use this rule of thumb: divide the altitude you need to descend by 300. So if you need to go from 7,000’ to 1,000’ thats 6000/300, which is 20. Start your descent 20 miles out (add a few miles to level off and configure at the bottom of descent). This provides for a roughly 3° descent, which is fairly typical.
Reducing power is pretty normal for descents. Some aircraft moreso than others. But it’s often advisable to reduce power and remain level to slow down a bit first, and hold that slower speed during the descent. Remember what I said above - pull the power, stay level with yoke backpressure for a bit as the speed drops, then as you approach the desired airspeed,
gently let the nose fall until you’re maintaining that speed (using the yoke to control the speed), then re-trim to hands-off once you’re at the speed. Adding flaps and gear can help, but you don’t want to do this too far out, or when you’re going faster than the limits of those.
