how to edit like kings and generals youtube channel ?

Given that there's like 10 different techniques in every shot of those longer documentaries, such questions can ever only be answered by pointing out specific clips and timecodes. Rick already provided soem general directions, but overall it's simply just a lot of work and probably a collaborative effort where different people specialize in doing specific segments. One guy does the maps, another the 3D animation, another takes care of the edit and overall flow and so on...

Mylenium

First, plan your movie by writing a detailed script and making a storyboard for every shot.

Second, gather the artwork, 3D models, and other assets you need to complete your first shot.

Create a composition inside After Effects using one of the industry standard Composition presets. 4K if you have the processing power or HD and use 24 or 23.976 fps as a frame rate. If you create the comps at 30fps, the render time will increase about 20%. If you want to use 60 fps, expect the render time to increase by about 120%. There is no need for any higher frame rates for most videos. Feature Films are almost all at 24 fps.

Now that you have your first comp setup for your first shot (notice I said shot, not sequence or scene), it is time to stack up your assets and start creating the animations or integrating footage. One of the first shots I saw in the only video I looked at was a combination of transitions where organic-looking holes were made in a paper graphic revealing still images below while a camera appeared to scan through a bunch of those papers. I would make each section a different comp, find match cuts that can be made, then edit the final "scene" in Premiere Pro. 

Another section of the video was a series of animated maps. The artwork style was consistent with the rest of the video, so there was some serious art direction going on in the project. Each part of the story that uses a map should be a separate comp. Again, you do the final editing in Premiere Pro.

I saw other scenes that had 3D models of characters marching. 3D is a whole different world. You can spend days or weeks creating a character, then days or weeks animating a few seconds of interaction with other characters. Then you can spend days or weeks adjusting the lighting and adding textures and other elements to the 3D scene. Often you render several passes that can be composited in an app line After Effects to complete the scene. That's why you see hundreds of names in the credits in animated movies. It is very labor intensive.

I hope this gives you some things to think about. No single tutorial series covers more than one or two of the dozens of different workflows I saw scrubbing through about the first 10 minutes of the episode I watched. When you have some specific questions about specific shots, come back with more details, and we will see if we can point you in the right direction.

I'm going to go for the big picture answer:  Film school or mentor with someone.

I've never heard of Kings and Generals.  I looked it up on YouTube, but there are literally hours and hours of clips.  

Please post a specific link, with a time marker, to an example of what SPECIFICALLY you mean.