The .dwgs are huge / are full of junk.

Its better to link the dwg in auto center?

That's the reason!!

in order to work with dwg's you need to link your dwg via the "Center to Center option" and then you need to acquire coordinates, but much before than that you need to clean deeply IN AUTOCAD with the (-PU ) Command, and other commands your dwg because already said by @Snowyweston dwg's are full of undesirable objects (junk)

Thank you for your answers!

So it is better every time to select auto Center-center, when I want to link dwg?
When I want to my place the project in the real coordinations I will use the option acquired coordinates?
This is a step that I need to do it when I will have finished the project as I will have the same project as now if I have the real coordinates?

Sorry I am confused with the process of linking and the coordinates.

The dwg files that you linked, probably, have massive extents. When you 'zoom to fit', it zooms out to cover the full extents which, in turn, take up most of the screen, leaving your revit objects very tiny on screen.

I'd suggest you do the following:

1. In the properties box for the 3D View, click on the 'Crop View' and the 'Crop Region Visible' check-boxes.

2. The crop region lines will appear on screen. Set ir up around the your model by dragging the lines and uncheck the 'Crop Region Visible' check-box.

3. This will help you to view only the required area when you 'zoom to fit'.

4. Remove the dwg files as soon as you are done with them and uncheck the 'Crop View' to get the entire workspace back on display.

I cannot understan I use the auto center to center and a link cad it is not visible in the floor and it seems that it is far away from the center of revit..

Question 1: YES!!

Question 2: Yes it can be quite confusing, and there are several techniques to do that, so I think you'll need to check in all RFO content, but briefly thats the way I'll do this
Question 3: All that depends the way you work, but definitively I'll tend to acquire coordinates before to continue my work,

BUT as already said dwg's are full of debris and undesirable things.

If you prefer we can do this in Spanish or French if you think that will be more easy for you! regards

that's because your dwg is plenty of trash and objects that are far from the origin and center, that's why you NEED to deeply clean you dwg file before linking it into your model

This is something I explain often.
When you open your DWG file in autocad, if you draw a line from the center of your project and type 0,0,0 do you end up with a really long line and your project is tiny? Draw a line, pick a point somewhere in your project, and then for the 2nd point just type 0,0,0 and enter. This will draw a line from your first point (on your project) to the Origin of Autocad which is 0,0,0 coordinate.

This is because the project in autocad has been "geopositioned" meaning its coordinates are geographically correct (based on your local benchmark - in america and in autocad as it is an american product, the default geo benchmark is Texas) You can redefine these coordinates later in civil3D or other programs.

But the simple answer is that your building has been located properly to its coordinates in your source DWG file.

When you link the dwg in revit (origin, origin) there is the same 0,0,0 (UCS) coordinate system as in autocad.
So the effect you have is the same as opening your autocad dwg file and drawing a line from center of project to 0,0,0.
Your point of origin is the same 0,0,0 UCS of autocad.
The problem is that revit is limited to a 33km radius, so often this will mean your project is way off to the side, and tiny when you zoom extents in revit. And in addition revit will warn you that you are outside of your 33km radius.

The best way to handle this is as follows.
- Insert dwg centre/centre.
- Go to the manage tab, and inherit the coordinates of the linked file (this wil move your "session" to the correct coordinates based on the coordinates in your source file (the dwg - which we are assuming has been geolocated correctly)
- After you do that, unpin the paper clip icon of your project base point, and move it to the corner of your project or intersection of two grid lines or whatever is your project base point and then REPIN the paperclip icon.

You will probably also find that if you zoom extents now, you will have the triangle (survey base point) and the circle (project base point) in different positions and your project will be tiny. Because your survey base point will stay at 0,0,0 and your project base point will be updated to the inhereted coordinates. Deactivate the survey base point in the VG options, and zoom extents again and you will be focused on your project location.

Now you can delete the linked cad file (ie remove the link entirely).
If you re-link the cad file now, use origin/origin. It will now link in the correct place, because your coordinates (and your working session) have been placed over the project and the 0,0,0 is still arbitrarily off somewhere way in the distance.

Linking the dwg file centre/centre actually puts the dwg file centred on your current work session position (in most cases in revit this will be 0,0,0 UCS) - for example if you were to draw the line in your autocad file from project to 0,0,0 UCS and save it with this line still there, the "centre" of the dwg file will now include that LONG line all the way back to UCS origin (not ideal). It takes the actual drawn elements in autocad and estimates a "center" point based on what is present in the source file and places it centered over your current work zone in revit. This may or may not be 0,0,0 in revit depending on whether you already modified coordinates in revit or not. This does not affect the coordinate system of the linked file, hence we can inherit the coordinates of the linked file to put ours in revit up to date.

Linking origin/origin links to the UCS 0,0,0 origin of both products.