[Need Advice/Tips] Landscape Painting/Creation

I’ve been trying to create a photorealistic landscape and I’m having some issues with it; can’t say what exactly, since it just doesn’t look that good.
I’m using layers blend, took a good (I think) surface texture ( from textures.com , if it matters), and set it on my material.

I was hoping people here can give tips/advice they have encountered with landscape.
Specifically (or rather, some things I’ve been trying to understand better):

  1. What to put in mapping scale? Some old thread ( https://forums.unrealengine.com/development-discussion/content-creation/1114-landscape-texture ) says to use the landscape size, but what is the size exactly? does that mean the “overall resolution”? Even if so, other places online suggested using a number between 0-10, which seems completely different. When to use which values?

  2. I found some online video of a landscape creation, and at one point he increased the height of an area (using the sculpt tool), and then somehow, the slope area became a different material; how can once do that?

  3. I know you can have up to 16, unless you set the textures sample source to Shared:Wrap. I know what wrap means, but what does Shared mean? what are the consequence of it? why isn’t it on by default?

Answers to these 3 would be great, and share more if you have!

… anyone?