How can I create realistic fur, feathers or hair without crashing UPBGE and slowing down the player? This would have to be made with either particles or geometry nodes. While UPBGE does have a Hair particle system, it only lets me make spiky hair, like what you find on baby birds known as “down” feathers, or just some spiky fluff like on a teddy bear. Can anyone maybe point me to some tutorials on how to do this? Like geometry node based fur/feathers or something else? Examine the fluff on a longhaired cat or the feathers on a chicken. You will see what I mean.
Even though interactive realistic fur would be nice, using this technology inside a video game is a waste of CPU power which, I guess, is very much needed if you want to make a realistic looking game with lots of objects, particles, PBR materials and stuff combined with the actual code of your game. So you are gonna have to cheat. That said, right now, animated geometry nodes based hair is a thing only in animation nodes, which is a free Blender Addon, meaning it might not work with UPBGE.
For classic not interactive hair, once you added your hair particle system, switch from object mode to particle edit and you’ll be able to groom it the way you like it.
For stylized fur, a trick I like to use is to just paint a hair spike with transparent background, use it as a texture on a plane, and instance it across your mesh using geometry nodes 
or a hair particle system ( just go to your Particle system setting → Render → Render As and set it to Object instead of Path. Then you just have to select your plane and TADAA )
Hope this helps
UPBGE has a parallax node that works as a visual displacement of the texture on the height maps - and has settings with which you set the displacement force and displacement level - by playing with these settings you can create an effect, fur, feathers, volume for smoke, water, glass, or create the illusion of object displacement on a completely flat mesh
follow the link to find the configuration of nodes with a parallax node and see what effect it gives when mixed with diffuse textures, normal map and other maps - if you have performance problems, connect a UV camera node and two math nodes to the parallax node, divide the first and multiply the second - this will give you a level of detail depending on the distance to cameras - when approaching, the detail will turn on and when moving away, it will turn off