Please dumbproof explanation how exactly to connect the node groups

The 1st question is simple - clicking on “new” materials and adding any kind of materials/texture adds a “Material” output node; but the groups which don’t have output node included have an output socket marked as BSDF and the Group output node has BSDF input. Is this working as “surface” input of the Material output node so I need just to connect both? And will this override already existing material/texture combo?
And the 2nd question - what when I add a group which already has a Material output node included (as I understood they shouldn’t but still some of the premade groups have such node included)? It overrides the rendered image but the material and the texture (if existing) remain the old one, which often causes a mess. I tried to add/delete different Outputs but this cause a mess again.
So is there a way to easily override this and to get material, texture and rendering from the group only?

material nodes output a color, shaded on a per pixel basis for bge real time or blender internal render. like any yellow socket, it is a like a texture, containing RGB channels. you can do alot of creative and strange things. but only one output can be “active”.

green sockets are shaders, data only readable by cycles render. absolutely no other color can connect them.

material, texture, and yellow output nodes are only for blender internal or bge.

bsdf and other green things and image nodes are only for cycles.

two different node setups can exist side by side, but there can only be a little overlap.

So my problem is in existing node groups which are made for the blender render, and if I use cycles render NOT to use yellow group outputs?

better yet, dont use node groups for the wrong engine. unless you figured out how they work. its easiest to work in absolutes until you learn what the exceptions are.

spend a day experimenting with one node at a time.