3D environment techniq and the right methods

Hi everyone,

this is my first topic, I am learning 3D modeling for games,
I start to love more in environment stuff and I love style like ape escape, Mario klonoa etc...

I know how to model props and texturing it. but I need to understand how do they built the environment levels like ape escape 2 and 3...

most what bother me is how do they blend between 2 object so that doesn't show seams.

and if there is big land and the land was Grass seamless then I want to put hole or some rust ground in the seamless grass how do I do it ?

like this example I don't know how resolve it...



I model this, and what's bother me this part how I solve it? what the good way ? :o

here examples from ape escape:












what the way of modeling environment for games :o
is the blend UV or what it call it "vertex color" are used alot in games ?
or there is another way?

I search alot for this topic all the videos talk about design props no one explain the methods even I didn't find a good course or book :o



...


thanks.