I am working as a 3D engineer on a non game related work, its a desktop rendering that renders and edit 3D stuff.
That means I am an experienced OpenGL dev and I know its core, vertex and fragment shaders, the maths (vector and matrix), etc. Before that I was a hobbyist game engine coder and built one though not high fidelity for OpenGL ES (mobile), I also know Directx 9.

Well, that reason for my background introduction, aside for being new to this site (my friend recommended it), is to let people know Im not a newbie to the 3D dev world but a relatively newbie to game/engine world.

I want to achieve this look in my target RTT game I am planning to do

Any Advice on how can I achieve this look?
1. how to render trees and grass efficiently
2. What type of shadow algo does it use (opinion is ok)
3. any tips/advice for water/lake/river rendering technique and some resources you guys can share?
4. how to render the ground, does it use some kind of texture splatting of some sort?
5. lighting and shading technique possible to achieve that look?

Any tips and advice will do
TIA

1. This screenshot looks like it's using billboards for a lot of foliage things which is fairly common (on the way out in AAA but still widely used). Basically just drawing a quad with a picture of grass or leaves or whatever on it that rotates to face the camera so it looks 3D-ish.
2. Almost definitely shadow mapping. Probably a single cascade, something to soften the edges (PCF maybe).
3. The water here is fairly primitive - just taking the colour behind the water, distorting it a bit for refraction, then colouring it blue based on the difference between the water's surface depth and the depth of the scene behind it. Maybe the order's different. It doesn't look like there are any reflections going on.
4. Probably they have a terrain system where an artist/level designer is able to paint textures onto a mesh. Often what happens in these types of systems is the painting is applied to one large texture that gets drawn to the entire terrain (or split into chunks if the terrain is large).
5. To me it just looks like simple phong shading with some normal mapping. I don't think there's any PBR here.