I am not even sure you'll see this reply, as I'm having trouble logging in. This site has been wonky for the last few days. I'll try to answer your questions as best I can.
The following is long as I don't know how to be concise, lol. It's thorough, though.
- I think the hangout is mostly here, though Mod the Sims has an active forum. As does Reddit, and Carl the Sims 4. Discord has some activity. But I think this site has the most active users.
- Simblr is still going strong. Patreon is where a lot of creators are these days, as well.
- Do not know. Probably YouTube, as you mentioned.
- This is something I'm looking into, as well. If you can do good work and gain a following, you can make some decent-ish money, I think.
- The biggest earners produce work that is professional-level. And by that, I mean the kind of content — I'm referring to CAS and build/buy items and not scripting — that EA, itself, produces. At a minimum, your work would need to be at least average, your screenshots would need to be attractive, you would need to produce content people actually want, and you would need to advertise.
- Regarding advertising, BE EVERYWHERE. X, Facebook, Instagram, Pinterest, YouTube, TikTok, Discord, email lists, etc. Saturate the market. Utilize effective hashtags and titles that effectively "hook" your downloaders. Start looking at content as a creator and not a consumer, and you'll see what others are doing to promote themselves.
- Regarding screenshots, good screenshots go a long way. You can take decent screenshots inside Blender (or whatever 3D modeling software you use), but include some in-game screenshots so people know what your content looks like in the game. If it's CAS items, take screenshots in the CAS section where you create your sim (can't remember what it's called) and in live play. Look at other creators' screenshots to get an idea of what works.
- Downloaders want something that looks good/works well in the game — no visual glitches, nothing so high-poly or such large textures that it makes the game lag, doesn't interfere with other CC, and so on. Many downloaders don't know enough about the creation process to know if something looks good "under the hood." In other words, they aren't going to open up a mesh in Blender to see if it's well-made, and they aren't going to examine a texture to see if it's sloppy, so there's a little leeway in that area, I think, but if you aren't at least somewhat conscientious of things, the work you produce will be garbage, and people will know it's garbage, and word will get out.
- Produce content regularly and fairly often. CAS items can be made more quickly than build/buy items, especially if your build/buy items are in a set, such as a living room set, for example. Look at how often others are uploading to get an idea.
- Bonus answer: About making money.
- The money seems to be from Patreon. As far as I know. You can lock content behind a paywall at Patreon, but only for about three weeks, or so. EA, and the Simming community as a whole, frowns on permanent paywalls. While your work is yours, you are, after all, using EA's assets to create your things — you can't say that your CC is 100% your work — so you don't have a legal right to permanently paywall your stuff. And, as I mentioned, it's considered bad manners, and word will get out, and you'll be hated. PLUS, people will purchase your things and then upload them for free elsewhere, and they will do this if they think you are greedy. And you can do nothing about it.
- The Sims Resource pays its artists, but you have to be submitting acceptable and timely work for a while to get "hired." Which means you will be working for free for a time.
- Curseforge pays its artists, I think. I don't know the details of that, however.
- Donations are not a big-money kind of thing, as far as I know. You would need to make it worthwhile for downloaders. They do not want to monetarily contribute if your work is garbage, and if you rarely upload.
- You may be able to utilize ad money, somehow. But, I think you'd need your own site for that, outside of Patreon and/or Tumblr. At the same time, you need to be where the Simmers are, or you will get no traffic to your site in the first place. People aren't going to spend time searching for you unless you're notable in some way (a well-known artist who produces excellent work).
- There are other ways to make money from Simming than producing downloadable content. You could, for example, do a WCIF account that is a one-stop area for the coolest content. Collect tutorials. Write tutorials. Make videos for YouTube. Look around at YouTube to get an idea. Anything that fulfills a need or provides entertainment will work.
- Another bonus answer: If you want to make CC, you should start with recolors and work up to 3D modeling. Or learn to make tuning mods. There are a variety of things you can do, but if you are an absolute beginner, start with retexturing, then move up to converting items (like from another game), and then you can move on to Blender. And I do recommend Blender, as it's free and most, if not all, Sims meshing tutorials are geared toward Blender. The place to go for learning to make CC is the Sims4Studio site. Mod the Sims has a lot, as well. I would not recommend YouTube. Many CC creators there teach all the wrong things, and then you have to go to Sims4Studio to fix things. YouTube is good for videos on using Blender and on making textures, in general, but stick to the ones produced by professionals. Again, too many creators teach the wrong things.
Well, I think I about covered everything!