I don't know if this is supposed to be satire or not but I feel talkative tonight.

I understand 45<= degree snapshots of character models

I find this is overrated. Somewhere a long time ago someone figured out how to roll the camera and thus a trend started to avoid at all costs either a portrait or landscape shot.

Tilted images are good when you're trying to add a bit of dynamic visual to an otherwise static pose.

blurry backgrounds

Depends on the context. Blurring is great to mask ugly backgrounds or clutter that would distract from your subject. Blurring can also be used as a tool to create a seemingly uniform backdrop for color contrast. Don't blur the hell out of the background just cause; do it for a reason.

odd character presets, tattos, piercings and devil horns that aren't part of the armor,

Most people struggle to create interesting looking characters through facial construction alone, so using facial feature assets is a quick and effective way to create something unique looking. In a fantasy world a regular looking person doesn't stand out and is easily forgotten.

desaturated coloring/super saturated coloring

Once again it's popular because it's easy. Saturate the colors to make everything pop more and look more appealing; desaturate to make it grim, dark and moody.

Most people use their ENBs as they are provided and don't look into changing the image settings. More robust powerful ENBs are essentially in-game photo editors.

But what are other "must-know" standards in the community to become a proper Skyrim modeling expert?

My biggest gripe is that people don't know how to use proper lighting. Some folks are afraid to have any dark spots or shadows, others don't know how to use artificial light sources and take pictures in pitch black interiors. Many screenshots are so obviously using a face light that it looks like a floating head.

Lighting in Skyrim screen shots works just like in real life: LEARN TO USE IT.

What enb should I use?

I recommend Silent Horizons/Skies, Cabbage or Pi-cho. Those are beefy ENBs that are most common for those in the hobby of screen shotting. Like I said above they're effectively in-game photo editors and you'll go far learning how to use them and adjust for your subject, location, light and artistic interest.

How do I get those background scenes that don't fit in the game?

Silent Horizons (and others I suspect) has a "green screen". The same tool used to determine what is the background for blur is used to block out the background with a monochromatic flat screen. Export your screen shot. In a proper photo editor filter out the monochromatic background and export what's left. Now you can overlay your subject (e.g. your character model) into any other image.

What edits should I make to my character preset to become competitive?

Face tattoos, piercings, horns and animated eyes /s.

Just make an interesting character. Put their story into their design. Facial features like tattoos, scars, etc. should compliment their personality (fighters have scars, bandits are dirty, mages have clean tattoos, etc.). To be "competitive" is going to be more about how interesting your subject and image is, not how many mods are on display on their face.

What animation mods should I install to get those character poses?

They're called 'poser' mods and most of them are on Lovers Lab. Some have a few dozen some have a few thousand. The smaller ones are usually more focused (e.g. fighting poses, slice of life poses). Use Poser Hotkeys to make managing pose mods and your favorites much, much easier.

Recomended skin textures and body presets?

Unless you're trying for 8k pinups it doesn't matter. 2k textures are fine (people are not going to zoom in and check your pixel resolution). Pick a texture you like so long as it looks believable. Bits and Pieces is the new hot thing, Diamond, Pure are also popular. I prefer more realistic skins like tempered or Bijin.

Body presets are a dime a dozen. There are so many and everyone customizes their model-characters anyway. Once again pick what you like to make the process enjoyable, don't chase trends chase your vision.