As a beginner, I want to (all) remove unnecessary vertices (automatically).
e.g. take a cube extrude one size now you have a rectangle with 4 vertices in the middle.
Actually these 4 vertices are unnecessary (when everything is perfect alligned of course).
How do I remove those vertices in editmode?
(Decimeter wasn’t ready helpfull or did I use incorrect?)
If i understand what your asking correctly, select the 4 vertices that make up the middle ring of the rectangle , press x and choose remove edge loop, but what you have then created is the same as just selecting the end face and moving it one size rather then exruding it.
If this is not what you are meaning, the vertices may be necessary to create the geometry because you can only make faces from either 3 or 4 vertices.
Is there know way to combine adjacent faces into 1 where possible?
The way you described is a manual job. Is there no automated process for that?
The extruding-cube example I gave is just to illustrate the problem when my primary goal is to end up with a rectangular cube resizing is the way to go.
With this example it is clear that the middle ring of vertices is superficial.
Currently this is no and no (in most cases that is). The answer why this is so is located here.
The only good way to selectively remove unnecessary faces/vertices/edges right now is to delete a edge-loop (Select an edge loop with [Alt]+[RMB on edge], press ->Edge Loop)