I intend to learn how to fly RC fixed-wing planes (I have previously worked on semi-autonomous aircraft, but had never learned how to fly RC planes). However, I currently live in a small place where there are no suitable places for flying planes (due to population density, lack of facilities, and laws). There are no "empty places" in the entire country. For now, I will be satisfied with flying in a simulator, leaving real-life practice for the future. I don't mind buying the RC remote control. What kind of simulator should I get? Is there any criteria I should use to find a suitable simulator for fixed-wing RC aircraft?
I've enjoyed the Great Planes RealFlight simulator for a number of years. It has lasted well and I like the included transmitter controller. It is not free nor cheap, but it will save you a lot more in RC models that you can bring home from the flying field ready to fly again (i.e. it will save you many crashes).
My go-to simulator is the free CurryKitten FPV Sim, but I’m biased as I helped with the asset design.
Another good option is RC Plane 3 on Steam, and it’s also free!
To control it, it’s recommended you get a hobby-grade transmitter. Most of these can simply plug into the computer and function as a joystick, though depending on the program you use you may have to map the axes.
Most people start in 'chase plane' mode, where the virtual camera follows the model. This lets you get a feel for the controls. Once you've got the basics, switch to a camera on the ground because the vital skill to learn is thinking of the control directions relative to the plane and not your view. The most common beginner mistake is to steer the wrong way when the plane is flying towards them, so practice this until steering the right way feels natural to you.
Your first challenge is just keeping the plane in the air and within sight. If you find this hard, pick a large, high-wing trainer plane, and reduce the simulation speed. Many sims have a 'zoom' function that helps when the plane is far away. Don't get too used to artificial aids like this though, turn them off as soon as you can manage without them because they won't be there in real life!
Then learn to land. Learn where the plane needs to be to glide in to the runway, and when to pull the nose up (flare) to touch down gently.
When you can land, turn on some wind to make it more realistic. Try a wide range of models to learn their differences. Practice flying an out-of-trim model because almost all models will be out of trim on their first flight, and you can even turn up the simulation speed to simulate how nervous you will be on your first real flight.
When you can do that, I suggest downloading the BMFA's achievement scheme notes (look in the Downloads menu) or similar for another country. Having a syllabus will give you a set of targets, so you learn the basics before moving on to aerobatics, and there's a lot of guidance on how to do everything.
Now for the opinion bit: I suggest trying a few free simulators, then downloading the demo version of Real Flight to see if the extra features are worthwhile. Free simulators are perfectly adequate for learning to fly but the extra graphics quality, models and game modes are more likely to keep you coming back to a commercial sim.
I also suggest buying a small toy quadcopter/drone that can be flown indoors. That will teach you fine stick control and you can learn to avoid control-reversal when the model is flying towards you. It will also teach you the one thing sims are bad at - flying carefully so you aren't crashing all the time!